diff --git a/src/libraries/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.Shared.xml b/src/libraries/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.Shared.xml
index f7e6fd1f2f0606..1730d7fe5b1291 100644
--- a/src/libraries/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.Shared.xml
+++ b/src/libraries/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.Shared.xml
@@ -1,13 +1,4 @@
 <linker>
-  <assembly fullname="System.Private.CoreLib">
-    <type fullname="System.Threading.ThreadPoolBoundHandle">
-      <!-- Workaround to keep .interfaceimpl even though this type
-             is not instantiated on unix:
-             https://github.com/mono/linker/pull/649 -->
-      <method name=".ctor" />
-    </type>
-  </assembly>
-
   <assembly fullname="System.Private.CoreLib" feature="System.Runtime.InteropServices.EnableConsumingManagedCodeFromNativeHosting" featurevalue="true">
     <!-- Native hosting accesses managed methods from
     https://github.com/dotnet/runtime/blob/bbc898f3e5678135b242faeb6eefd8b24bf04f3c/src/native/corehost/hostpolicy/hostpolicy.cpp#L527-L538
diff --git a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
index cf031056c51f8f..ece38286e289a0 100644
--- a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
+++ b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
@@ -1264,7 +1264,6 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\CompressedStack.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Condition.cs" Condition="'$(FeatureMono)' != 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\StackCrawlMark.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\DeferredDisposableLifetime.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\EventResetMode.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\EventWaitHandle.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ExecutionContext.cs" />
@@ -1344,9 +1343,6 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\WaitHandle.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\WaitHandleCannotBeOpenedException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\WaitHandleExtensions.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Win32ThreadPoolNativeOverlapped.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Win32ThreadPoolNativeOverlapped.ExecutionContextCallbackArgs.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Win32ThreadPoolNativeOverlapped.OverlappedData.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\ThreadStaticAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\ThrowHelper.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\TimeOnly.cs" />
@@ -2350,18 +2346,23 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\RuntimeInformation.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\StandardOleMarshalObject.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Security\SecureString.Windows.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\DeferredDisposableLifetime.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\EventWaitHandle.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\LowLevelMonitor.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Mutex.Windows.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PreAllocatedOverlapped.WindowsThreadPool.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\RegisteredWaitHandle.WindowsThreadPool.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Thread.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Semaphore.Windows.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\TimerQueue.Windows.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\TimerQueue.WindowsThreadPool.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Thread.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPool.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolBoundHandle.WindowsThreadPool.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PreAllocatedOverlapped.WindowsThreadPool.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolBoundHandleOverlapped.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\TimerQueue.Windows.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\TimerQueue.WindowsThreadPool.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\WaitHandle.Windows.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Win32ThreadPoolNativeOverlapped.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Win32ThreadPoolNativeOverlapped.ExecutionContextCallbackArgs.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Win32ThreadPoolNativeOverlapped.OverlappedData.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\WindowsThreadPool.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\TimeZoneInfo.Win32.cs" />
   </ItemGroup>
@@ -2855,7 +2856,7 @@
   <ItemGroup Condition="'$(FeaturePortableThreadPool)' == 'true'">
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\CompleteWaitThreadPoolWorkItem.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPool.Portable.Windows.cs" Condition="'$(TargetsWindows)' == 'true'" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPool.Unix.cs" Condition="'$(TargetsUnix)' == 'true' or '$(TargetsBrowser)' == 'true' or '$(TargetsWasi)' == 'true'" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPool.Unix.cs" Condition="'$(TargetsWindows)' != 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PortableThreadPool.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PortableThreadPool.Blocking.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PortableThreadPool.GateThread.cs" />
@@ -2866,27 +2867,24 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PortableThreadPool.WaitThread.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PortableThreadPool.WorkerThread.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PortableThreadPool.WorkerTracking.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PortableThreadPool.Unix.cs" Condition="'$(TargetsUnix)' == 'true' or ('$(TargetsBrowser)' == 'true' and '$(FeatureMultithreading)' != 'true') or '$(TargetsWasi)' == 'true'" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PortableThreadPool.Unix.cs" Condition="'$(TargetsWindows)' != 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PortableThreadPool.Windows.cs" Condition="'$(TargetsWindows)' == 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\LowLevelLifoSemaphore.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\LowLevelLifoSemaphore.Windows.cs" Condition="'$(TargetsWindows)' == 'true'" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\LowLevelLifoSemaphore.Unix.cs" Condition="'$(TargetsUnix)' == 'true' or '$(TargetsBrowser)' == 'true' or '$(TargetsWasi)' == 'true'"  />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\LowLevelLifoSemaphore.Unix.cs" Condition="'$(TargetsWindows)' != 'true'" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PreAllocatedOverlapped.Portable.cs" Condition="'$(TargetsWindows)' == 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PreAllocatedOverlapped.Windows.cs" Condition="'$(TargetsWindows)' == 'true'" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PreAllocatedOverlapped.Unix.cs" Condition="'$(TargetsUnix)' == 'true' or ('$(TargetsBrowser)' == 'true' and '$(FeatureMultithreading)' == 'true')" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PreAllocatedOverlapped.Portable.cs" Condition="'$(FeatureMultithreading)' == 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\RegisteredWaitHandle.Windows.cs" Condition="'$(TargetsWindows)' == 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\RegisteredWaitHandle.Unix.cs" Condition="'$(TargetsWindows)' != 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\RegisteredWaitHandle.Portable.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolBoundHandle.Portable.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolBoundHandle.Unix.cs" Condition="'$(TargetsWindows)' != 'true'" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolBoundHandle.Portable.cs" Condition="'$(TargetsWindows)' == 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolBoundHandle.Windows.cs" Condition="'$(TargetsWindows)' == 'true'" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolBoundHandleOverlapped.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolCallbackWrapper.cs" Condition="'$(FeatureNativeAot)' != 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Backoff.cs" />
   </ItemGroup>
-  <ItemGroup Condition="'$(FeatureMultithreading)' != 'true'">
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PreAllocatedOverlapped.Browser.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolBoundHandle.Browser.cs" />
+  <ItemGroup Condition="'$(TargetsWindows)' != 'true'">
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\PreAllocatedOverlapped.PlatformNotSupported.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPoolBoundHandle.PlatformNotSupported.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetsBrowser)' == 'true' and '$(FeatureMultithreading)' != 'true'">
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPool.Browser.cs" />
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/Overlapped.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/Overlapped.cs
index 423e9429b33be0..13b8eec743e5fc 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/Overlapped.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Threading/Overlapped.cs
@@ -68,12 +68,10 @@ public IntPtr EventHandleIntPtr
 
         [Obsolete("This overload is not safe and has been deprecated. Use Pack(IOCompletionCallback?, object?) instead.")]
         [CLSCompliant(false)]
-        [RequiresUnsafe]
         public NativeOverlapped* Pack(IOCompletionCallback? iocb)
             => Pack(iocb, null);
 
         [CLSCompliant(false)]
-        [RequiresUnsafe]
         public NativeOverlapped* Pack(IOCompletionCallback? iocb, object? userData)
         {
             if (_pNativeOverlapped != null)
@@ -95,12 +93,10 @@ public IntPtr EventHandleIntPtr
 
         [Obsolete("This overload is not safe and has been deprecated. Use UnsafePack(IOCompletionCallback?, object?) instead.")]
         [CLSCompliant(false)]
-        [RequiresUnsafe]
         public NativeOverlapped* UnsafePack(IOCompletionCallback? iocb)
             => UnsafePack(iocb, null);
 
         [CLSCompliant(false)]
-        [RequiresUnsafe]
         public NativeOverlapped* UnsafePack(IOCompletionCallback? iocb, object? userData)
         {
             if (_pNativeOverlapped != null)
@@ -134,7 +130,6 @@ public static void Free(NativeOverlapped* nativeOverlappedPtr)
             FreeNativeOverlapped(nativeOverlappedPtr);
         }
 
-        [RequiresUnsafe]
         private NativeOverlapped* AllocateNativeOverlapped(object? userData)
         {
             NativeOverlapped* pNativeOverlapped = null;
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/PreAllocatedOverlapped.Browser.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/PreAllocatedOverlapped.Browser.cs
deleted file mode 100644
index caad4d140aa403..00000000000000
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/PreAllocatedOverlapped.Browser.cs
+++ /dev/null
@@ -1,14 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-namespace System.Threading
-{
-    public sealed class PreAllocatedOverlapped : System.IDisposable
-    {
-        [CLSCompliantAttribute(false)]
-        public PreAllocatedOverlapped(IOCompletionCallback callback, object? state, object? pinData) { }
-        [CLSCompliantAttribute(false)]
-        public static PreAllocatedOverlapped UnsafeCreate(IOCompletionCallback callback, object? state, object? pinData) => new PreAllocatedOverlapped(callback, state, pinData);
-        public void Dispose() { }
-    }
-}
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/PreAllocatedOverlapped.PlatformNotSupported.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/PreAllocatedOverlapped.PlatformNotSupported.cs
new file mode 100644
index 00000000000000..5ce63f8a8736c7
--- /dev/null
+++ b/src/libraries/System.Private.CoreLib/src/System/Threading/PreAllocatedOverlapped.PlatformNotSupported.cs
@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Threading
+{
+    public sealed class PreAllocatedOverlapped : IDisposable
+    {
+        [CLSCompliant(false)]
+        public PreAllocatedOverlapped(IOCompletionCallback callback, object? state, object? pinData)
+        {
+            ArgumentNullException.ThrowIfNull(callback);
+            throw new PlatformNotSupportedException(SR.PlatformNotSupported_OverlappedIO);
+        }
+
+        [CLSCompliant(false)]
+        public static PreAllocatedOverlapped UnsafeCreate(IOCompletionCallback callback, object? state, object? pinData)
+        {
+            ArgumentNullException.ThrowIfNull(callback);
+            throw new PlatformNotSupportedException(SR.PlatformNotSupported_OverlappedIO);
+        }
+
+        public void Dispose() { }
+    }
+}
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/PreAllocatedOverlapped.Unix.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/PreAllocatedOverlapped.Unix.cs
deleted file mode 100644
index d638e6773f2cef..00000000000000
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/PreAllocatedOverlapped.Unix.cs
+++ /dev/null
@@ -1,124 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-using System.Diagnostics;
-
-namespace System.Threading
-{
-    /// <summary>
-    /// Represents pre-allocated state for native overlapped I/O operations.
-    /// </summary>
-    /// <seealso cref="ThreadPoolBoundHandle.AllocateNativeOverlapped(PreAllocatedOverlapped)"/>
-    public sealed partial class PreAllocatedOverlapped : IDisposable, IDeferredDisposable
-    {
-        /// <summary>
-        ///     Initializes a new instance of the <see cref="PreAllocatedOverlapped"/> class, specifying
-        ///     a delegate that is invoked when each asynchronous I/O operation is complete, a user-provided
-        ///     object providing context, and managed objects that serve as buffers.
-        /// </summary>
-        /// <param name="callback">
-        ///     An <see cref="IOCompletionCallback"/> delegate that represents the callback method
-        ///     invoked when each asynchronous I/O operation completes.
-        /// </param>
-        /// <param name="state">
-        ///     A user-provided object that distinguishes <see cref="NativeOverlapped"/> instance produced from this
-        ///     object from other <see cref="NativeOverlapped"/> instances. Can be <see langword="null"/>.
-        /// </param>
-        /// <param name="pinData">
-        ///     An object or array of objects representing the input or output buffer for the operations. Each
-        ///     object represents a buffer, for example an array of bytes.  Can be <see langword="null"/>.
-        /// </param>
-        /// <remarks>
-        ///     The new <see cref="PreAllocatedOverlapped"/> instance can be passed to
-        ///     <see cref="ThreadPoolBoundHandle.AllocateNativeOverlapped(PreAllocatedOverlapped)"/>, to produce
-        ///     a <see cref="NativeOverlapped"/> instance that can be passed to the operating system in overlapped
-        ///     I/O operations.  A single <see cref="PreAllocatedOverlapped"/> instance can only be used for
-        ///     a single native I/O operation at a time.  However, the state stored in the <see cref="PreAllocatedOverlapped"/>
-        ///     instance can be reused for subsequent native operations.
-        ///     <note>
-        ///         The buffers specified in <paramref name="pinData"/> are pinned until <see cref="Dispose"/> is called.
-        ///     </note>
-        /// </remarks>
-        /// <exception cref="ArgumentNullException">
-        ///     <paramref name="callback"/> is <see langword="null"/>.
-        /// </exception>
-        /// <exception cref="ObjectDisposedException">
-        ///     This method was called after the <see cref="ThreadPoolBoundHandle"/> was disposed.
-        /// </exception>
-        [CLSCompliant(false)]
-        public PreAllocatedOverlapped(IOCompletionCallback callback, object? state, object? pinData) :
-            this(callback, state, pinData, flowExecutionContext: true)
-        {
-        }
-
-        private DeferredDisposableLifetime<PreAllocatedOverlapped> _lifetime;
-
-        /// <summary>
-        ///     Initializes a new instance of the <see cref="PreAllocatedOverlapped"/> class, specifying
-        ///     a delegate that is invoked when each asynchronous I/O operation is complete, a user-provided
-        ///     object providing context, and managed objects that serve as buffers.
-        /// </summary>
-        /// <param name="callback">
-        ///     An <see cref="IOCompletionCallback"/> delegate that represents the callback method
-        ///     invoked when each asynchronous I/O operation completes.
-        /// </param>
-        /// <param name="state">
-        ///     A user-provided object that distinguishes <see cref="NativeOverlapped"/> instance produced from this
-        ///     object from other <see cref="NativeOverlapped"/> instances. Can be <see langword="null"/>.
-        /// </param>
-        /// <param name="pinData">
-        ///     An object or array of objects representing the input or output buffer for the operations. Each
-        ///     object represents a buffer, for example an array of bytes.  Can be <see langword="null"/>.
-        /// </param>
-        /// <remarks>
-        ///     The new <see cref="PreAllocatedOverlapped"/> instance can be passed to
-        ///     <see cref="ThreadPoolBoundHandle.AllocateNativeOverlapped(PreAllocatedOverlapped)"/>, to produce
-        ///     a <see cref="NativeOverlapped"/> instance that can be passed to the operating system in overlapped
-        ///     I/O operations.  A single <see cref="PreAllocatedOverlapped"/> instance can only be used for
-        ///     a single native I/O operation at a time.  However, the state stored in the <see cref="PreAllocatedOverlapped"/>
-        ///     instance can be reused for subsequent native operations. ExecutionContext is not flowed to the invocation
-        ///     of the callback.
-        ///     <note>
-        ///         The buffers specified in <paramref name="pinData"/> are pinned until <see cref="Dispose"/> is called.
-        ///     </note>
-        /// </remarks>
-        /// <exception cref="ArgumentNullException">
-        ///     <paramref name="callback"/> is <see langword="null"/>.
-        /// </exception>
-        /// <exception cref="ObjectDisposedException">
-        ///     This method was called after the <see cref="ThreadPoolBoundHandle"/> was disposed.
-        /// </exception>
-        [CLSCompliant(false)]
-        public static PreAllocatedOverlapped UnsafeCreate(IOCompletionCallback callback, object? state, object? pinData) =>
-            UnsafeCreatePortableCore(callback, state, pinData);
-
-        private unsafe PreAllocatedOverlapped(IOCompletionCallback callback, object? state, object? pinData, bool flowExecutionContext)
-        {
-            // This construction is duplicated in PreAllocatedOverlapped.Windows.cs
-            // It has to either be duplicated or remove the 'readonly' part of _overlappedPortableCore
-            ArgumentNullException.ThrowIfNull(callback);
-
-            _overlappedPortableCore = new ThreadPoolBoundHandleOverlapped(callback, state, pinData, this, flowExecutionContext);
-        }
-
-        internal bool AddRef() => AddRefPortableCore();
-
-        /// <summary>
-        /// Frees the resources associated with this <see cref="PreAllocatedOverlapped"/> instance.
-        /// </summary>
-        public void Dispose()
-        {
-            DisposePortableCore();
-        }
-
-        internal void Release()
-        {
-            ReleasePortableCore();
-        }
-
-        unsafe void IDeferredDisposable.OnFinalRelease(bool disposed)
-        {
-            IDeferredDisposableOnFinalReleasePortableCore(disposed);
-        }
-    }
-}
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Browser.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.PlatformNotSupported.cs
similarity index 97%
rename from src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Browser.cs
rename to src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.PlatformNotSupported.cs
index 8e2f33e002f6b7..ab99e8eaad2468 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Browser.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.PlatformNotSupported.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
 using System.Runtime.InteropServices;
 
 namespace System.Threading
@@ -11,6 +12,7 @@ public sealed class ThreadPoolBoundHandle : IDisposable
 
         private ThreadPoolBoundHandle()
         {
+            Debug.Fail("Unreachable");
         }
 
         public static ThreadPoolBoundHandle BindHandle(SafeHandle handle)
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Portable.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Portable.cs
index 60a9c1848cc243..030092d9d6e16e 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Portable.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Portable.cs
@@ -17,15 +17,12 @@ namespace System.Threading
     /// </summary>
     public sealed partial class ThreadPoolBoundHandle : IDisposable
     {
-        [RequiresUnsafe]
         private unsafe NativeOverlapped* AllocateNativeOverlappedPortableCore(IOCompletionCallback callback, object? state, object? pinData) =>
             AllocateNativeOverlappedPortableCore(callback, state, pinData, flowExecutionContext: true);
 
-        [RequiresUnsafe]
         private unsafe NativeOverlapped* UnsafeAllocateNativeOverlappedPortableCore(IOCompletionCallback callback, object? state, object? pinData) =>
             AllocateNativeOverlappedPortableCore(callback, state, pinData, flowExecutionContext: false);
 
-        [RequiresUnsafe]
         private unsafe NativeOverlapped* AllocateNativeOverlappedPortableCore(IOCompletionCallback callback, object? state, object? pinData, bool flowExecutionContext)
         {
             ArgumentNullException.ThrowIfNull(callback);
@@ -36,7 +33,6 @@ public sealed partial class ThreadPoolBoundHandle : IDisposable
             return overlapped._nativeOverlapped;
         }
 
-        [RequiresUnsafe]
         private unsafe NativeOverlapped* AllocateNativeOverlappedPortableCore(PreAllocatedOverlapped preAllocated)
         {
             ArgumentNullException.ThrowIfNull(preAllocated);
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Unix.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Unix.cs
deleted file mode 100644
index 4235ebd50624e3..00000000000000
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.Unix.cs
+++ /dev/null
@@ -1,261 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-using System.Diagnostics;
-using System.Diagnostics.CodeAnalysis;
-using System.Runtime.InteropServices;
-
-namespace System.Threading
-{
-    //
-    // Implementation of ThreadPoolBoundHandle that sits on top of the CLR's ThreadPool and Overlapped infrastructure
-    //
-
-    /// <summary>
-    ///     Represents an I/O handle that is bound to the system thread pool and enables low-level
-    ///     components to receive notifications for asynchronous I/O operations.
-    /// </summary>
-    public sealed partial class ThreadPoolBoundHandle : IDisposable
-    {
-        private readonly SafeHandle _handle;
-        private bool _isDisposed;
-
-        private ThreadPoolBoundHandle(SafeHandle handle)
-        {
-            _handle = handle;
-        }
-
-        /// <summary>
-        ///     Gets the bound operating system handle.
-        /// </summary>
-        /// <value>
-        ///     A <see cref="SafeHandle"/> object that holds the bound operating system handle.
-        /// </value>
-        public SafeHandle Handle => _handle;
-
-        /// <summary>
-        ///     Returns a <see cref="ThreadPoolBoundHandle"/> for the specific handle,
-        ///     which is bound to the system thread pool.
-        /// </summary>
-        /// <param name="handle">
-        ///     A <see cref="SafeHandle"/> object that holds the operating system handle. The
-        ///     handle must have been opened for overlapped I/O on the unmanaged side.
-        /// </param>
-        /// <returns>
-        ///     <see cref="ThreadPoolBoundHandle"/> for <paramref name="handle"/>, which
-        ///     is bound to the system thread pool.
-        /// </returns>
-        /// <exception cref="ArgumentNullException">
-        ///     <paramref name="handle"/> is <see langword="null"/>.
-        /// </exception>
-        /// <exception cref="ArgumentException">
-        ///     <paramref name="handle"/> has been disposed.
-        ///     <para>
-        ///         -or-
-        ///     </para>
-        ///     <paramref name="handle"/> does not refer to a valid I/O handle.
-        ///     <para>
-        ///         -or-
-        ///     </para>
-        ///     <paramref name="handle"/> refers to a handle that has not been opened
-        ///     for overlapped I/O.
-        ///     <para>
-        ///         -or-
-        ///     </para>
-        ///     <paramref name="handle"/> refers to a handle that has already been bound.
-        /// </exception>
-        /// <remarks>
-        ///     This method should be called once per handle.
-        ///     <para>
-        ///         -or-
-        ///     </para>
-        ///     <see cref="ThreadPoolBoundHandle"/> does not take ownership of <paramref name="handle"/>,
-        ///     it remains the responsibility of the caller to call <see cref="SafeHandle.Dispose()"/>.
-        /// </remarks>
-        public static ThreadPoolBoundHandle BindHandle(SafeHandle handle)
-        {
-            ArgumentNullException.ThrowIfNull(handle);
-
-            if (handle.IsClosed || handle.IsInvalid)
-                throw new ArgumentException(SR.Argument_InvalidHandle, nameof(handle));
-
-            throw new PlatformNotSupportedException(SR.PlatformNotSupported_OverlappedIO);
-        }
-
-        /// <summary>
-        ///     Returns an unmanaged pointer to a <see cref="NativeOverlapped"/> structure, specifying
-        ///     a delegate that is invoked when the asynchronous I/O operation is complete, a user-provided
-        ///     object providing context, and managed objects that serve as buffers.
-        /// </summary>
-        /// <param name="callback">
-        ///     An <see cref="IOCompletionCallback"/> delegate that represents the callback method
-        ///     invoked when the asynchronous I/O operation completes.
-        /// </param>
-        /// <param name="state">
-        ///     A user-provided object that distinguishes this <see cref="NativeOverlapped"/> from other
-        ///     <see cref="NativeOverlapped"/> instances. Can be <see langword="null"/>.
-        /// </param>
-        /// <param name="pinData">
-        ///     An object or array of objects representing the input or output buffer for the operation. Each
-        ///     object represents a buffer, for example an array of bytes.  Can be <see langword="null"/>.
-        /// </param>
-        /// <returns>
-        ///     An unmanaged pointer to a <see cref="NativeOverlapped"/> structure.
-        /// </returns>
-        /// <remarks>
-        ///     <para>
-        ///         The unmanaged pointer returned by this method can be passed to the operating system in
-        ///         overlapped I/O operations. The <see cref="NativeOverlapped"/> structure is fixed in
-        ///         physical memory until <see cref="FreeNativeOverlapped(NativeOverlapped*)"/> is called.
-        ///     </para>
-        ///     <para>
-        ///         The buffer or buffers specified in <paramref name="pinData"/> must be the same as those passed
-        ///         to the unmanaged operating system function that performs the asynchronous I/O.
-        ///     </para>
-        ///     <note>
-        ///         The buffers specified in <paramref name="pinData"/> are pinned for the duration of
-        ///         the I/O operation.
-        ///     </note>
-        /// </remarks>
-        /// <exception cref="ArgumentNullException">
-        ///     <paramref name="callback"/> is <see langword="null"/>.
-        /// </exception>
-        /// <exception cref="ObjectDisposedException">
-        ///     This method was called after the <see cref="ThreadPoolBoundHandle"/> was disposed.
-        /// </exception>
-        [CLSCompliant(false)]
-        [RequiresUnsafe]
-        public unsafe NativeOverlapped* AllocateNativeOverlapped(IOCompletionCallback callback, object? state, object? pinData) =>
-            AllocateNativeOverlappedPortableCore(callback, state, pinData);
-
-        /// <summary>
-        ///     Returns an unmanaged pointer to a <see cref="NativeOverlapped"/> structure, specifying
-        ///     a delegate that is invoked when the asynchronous I/O operation is complete, a user-provided
-        ///     object providing context, and managed objects that serve as buffers.
-        /// </summary>
-        /// <param name="callback">
-        ///     An <see cref="IOCompletionCallback"/> delegate that represents the callback method
-        ///     invoked when the asynchronous I/O operation completes.
-        /// </param>
-        /// <param name="state">
-        ///     A user-provided object that distinguishes this <see cref="NativeOverlapped"/> from other
-        ///     <see cref="NativeOverlapped"/> instances. Can be <see langword="null"/>.
-        /// </param>
-        /// <param name="pinData">
-        ///     An object or array of objects representing the input or output buffer for the operation. Each
-        ///     object represents a buffer, for example an array of bytes.  Can be <see langword="null"/>.
-        /// </param>
-        /// <returns>
-        ///     An unmanaged pointer to a <see cref="NativeOverlapped"/> structure.
-        /// </returns>
-        /// <remarks>
-        ///     <para>
-        ///         The unmanaged pointer returned by this method can be passed to the operating system in
-        ///         overlapped I/O operations. The <see cref="NativeOverlapped"/> structure is fixed in
-        ///         physical memory until <see cref="FreeNativeOverlapped(NativeOverlapped*)"/> is called.
-        ///     </para>
-        ///     <para>
-        ///         The buffer or buffers specified in <paramref name="pinData"/> must be the same as those passed
-        ///         to the unmanaged operating system function that performs the asynchronous I/O.
-        ///     </para>
-        ///     <para>
-        ///         <see cref="ExecutionContext"/> is not flowed to the invocation of the callback.
-        ///     </para>
-        ///     <note>
-        ///         The buffers specified in <paramref name="pinData"/> are pinned for the duration of
-        ///         the I/O operation.
-        ///     </note>
-        /// </remarks>
-        /// <exception cref="ArgumentNullException">
-        ///     <paramref name="callback"/> is <see langword="null"/>.
-        /// </exception>
-        /// <exception cref="ObjectDisposedException">
-        ///     This method was called after the <see cref="ThreadPoolBoundHandle"/> was disposed.
-        /// </exception>
-        [CLSCompliant(false)]
-        [RequiresUnsafe]
-        public unsafe NativeOverlapped* UnsafeAllocateNativeOverlapped(IOCompletionCallback callback, object? state, object? pinData) =>
-            UnsafeAllocateNativeOverlappedPortableCore(callback, state, pinData);
-
-        /// <summary>
-        ///     Returns an unmanaged pointer to a <see cref="NativeOverlapped"/> structure, using the callback,
-        ///     state, and buffers associated with the specified <see cref="PreAllocatedOverlapped"/> object.
-        /// </summary>
-        /// <param name="preAllocated">
-        ///     A <see cref="PreAllocatedOverlapped"/> object from which to create the NativeOverlapped pointer.
-        /// </param>
-        /// <returns>
-        ///     An unmanaged pointer to a <see cref="NativeOverlapped"/> structure.
-        /// </returns>
-        /// <remarks>
-        ///     <para>
-        ///         The unmanaged pointer returned by this method can be passed to the operating system in
-        ///         overlapped I/O operations. The <see cref="NativeOverlapped"/> structure is fixed in
-        ///         physical memory until <see cref="FreeNativeOverlapped(NativeOverlapped*)"/> is called.
-        ///     </para>
-        /// </remarks>
-        /// <exception cref="ArgumentNullException">
-        ///     <paramref name="preAllocated"/> is <see langword="null"/>.
-        /// </exception>
-        /// <exception cref="ArgumentException">
-        ///     <paramref name="preAllocated"/> is currently in use for another I/O operation.
-        /// </exception>
-        /// <exception cref="ObjectDisposedException">
-        ///     This method was called after the <see cref="ThreadPoolBoundHandle"/> was disposed, or
-        ///     this method was called after <paramref name="preAllocated"/> was disposed.
-        /// </exception>
-        /// <seealso cref="PreAllocatedOverlapped"/>
-        [CLSCompliant(false)]
-        [RequiresUnsafe]
-        public unsafe NativeOverlapped* AllocateNativeOverlapped(PreAllocatedOverlapped preAllocated) => AllocateNativeOverlappedPortableCore(preAllocated);
-
-        /// <summary>
-        ///     Frees the unmanaged memory associated with a <see cref="NativeOverlapped"/> structure
-        ///     allocated by the <see cref="AllocateNativeOverlapped"/> method.
-        /// </summary>
-        /// <param name="overlapped">
-        ///     An unmanaged pointer to the <see cref="NativeOverlapped"/> structure to be freed.
-        /// </param>
-        /// <remarks>
-        ///     <note type="caution">
-        ///         You must call the <see cref="FreeNativeOverlapped(NativeOverlapped*)"/> method exactly once
-        ///         on every <see cref="NativeOverlapped"/> unmanaged pointer allocated using the
-        ///         <see cref="AllocateNativeOverlapped"/> method.
-        ///         If you do not call the <see cref="FreeNativeOverlapped(NativeOverlapped*)"/> method, you will
-        ///         leak memory. If you call the <see cref="FreeNativeOverlapped(NativeOverlapped*)"/> method more
-        ///         than once on the same <see cref="NativeOverlapped"/> unmanaged pointer, memory will be corrupted.
-        ///     </note>
-        /// </remarks>
-        /// <exception cref="ArgumentNullException">
-        ///     <paramref name="overlapped"/> is <see langword="null"/>.
-        /// </exception>
-        /// <exception cref="ObjectDisposedException">
-        ///     This method was called after the <see cref="ThreadPoolBoundHandle"/> was disposed.
-        /// </exception>
-        [CLSCompliant(false)]
-        [RequiresUnsafe]
-        public unsafe void FreeNativeOverlapped(NativeOverlapped* overlapped) => FreeNativeOverlappedPortableCore(overlapped);
-
-        /// <summary>
-        ///     Returns the user-provided object specified when the <see cref="NativeOverlapped"/> instance was
-        ///     allocated using the <see cref="AllocateNativeOverlapped(IOCompletionCallback, object, object)"/>.
-        /// </summary>
-        /// <param name="overlapped">
-        ///     An unmanaged pointer to the <see cref="NativeOverlapped"/> structure from which to return the
-        ///     associated user-provided object.
-        /// </param>
-        /// <returns>
-        ///     A user-provided object that distinguishes this <see cref="NativeOverlapped"/>
-        ///     from other <see cref="NativeOverlapped"/> instances, otherwise, <see langword="null"/> if one was
-        ///     not specified when the instance was allocated using <see cref="AllocateNativeOverlapped"/>.
-        /// </returns>
-        /// <exception cref="ArgumentNullException">
-        ///     <paramref name="overlapped"/> is <see langword="null"/>.
-        /// </exception>
-        [CLSCompliant(false)]
-        [RequiresUnsafe]
-        public static unsafe object? GetNativeOverlappedState(NativeOverlapped* overlapped) => GetNativeOverlappedStatePortableCore(overlapped);
-
-        public void Dispose() => DisposePortableCore();
-    }
-}
diff --git a/src/libraries/System.Threading.Overlapped/tests/ThreadPoolBoundHandle_PreAllocatedOverlappedTests.cs b/src/libraries/System.Threading.Overlapped/tests/ThreadPoolBoundHandle_PreAllocatedOverlappedTests.cs
index 728732326e5bae..0a116f2d0d33fd 100644
--- a/src/libraries/System.Threading.Overlapped/tests/ThreadPoolBoundHandle_PreAllocatedOverlappedTests.cs
+++ b/src/libraries/System.Threading.Overlapped/tests/ThreadPoolBoundHandle_PreAllocatedOverlappedTests.cs
@@ -9,7 +9,6 @@
 public partial class ThreadPoolBoundHandleTests
 {
     [Fact]
-    [ActiveIssue("https://github.com/mono/mono/issues/15313", TestRuntimes.Mono)]
     public unsafe void PreAllocatedOverlapped_NullAsCallback_ThrowsArgumentNullException()
     {
         AssertExtensions.Throws<ArgumentNullException>("callback", () => new PreAllocatedOverlapped(null, new object(), new byte[256]));
@@ -21,6 +20,7 @@ public unsafe void PreAllocatedOverlapped_NullAsCallback_ThrowsArgumentNullExcep
     }
 
     [Fact]
+    [PlatformSpecific(TestPlatforms.Windows)] // PreAllocatedOverlapped is not supported on non-Windows
     public unsafe void PreAllocatedOverlapped_NullAsContext_DoesNotThrow()
     {
         using (new PreAllocatedOverlapped((_, __, ___) => { }, (object)null, new byte[256])) { }
@@ -28,6 +28,7 @@ public unsafe void PreAllocatedOverlapped_NullAsContext_DoesNotThrow()
     }
 
     [Fact]
+    [PlatformSpecific(TestPlatforms.Windows)] // PreAllocatedOverlapped is not supported on non-Windows
     public unsafe void PreAllocatedOverlapped_NullAsPinData_DoesNotThrow()
     {
         using (new PreAllocatedOverlapped((_, __, ___) => { }, new object(), (byte[])null)) { }
@@ -35,6 +36,7 @@ public unsafe void PreAllocatedOverlapped_NullAsPinData_DoesNotThrow()
     }
 
     [Fact]
+    [PlatformSpecific(TestPlatforms.Windows)] // PreAllocatedOverlapped is not supported on non-Windows
     public unsafe void PreAllocatedOverlapped_EmptyArrayAsPinData_DoesNotThrow()
     {
         using (new PreAllocatedOverlapped((_, __, ___) => { }, new object(), new byte[0])) { }
@@ -42,6 +44,7 @@ public unsafe void PreAllocatedOverlapped_EmptyArrayAsPinData_DoesNotThrow()
     }
 
     [Fact]
+    [PlatformSpecific(TestPlatforms.Windows)] // PreAllocatedOverlapped is not supported on non-Windows
     [ActiveIssue("https://github.com/mono/mono/issues/15313", TestRuntimes.Mono)]
     public unsafe void PreAllocatedOverlapped_NonBlittableTypeAsPinData_Throws()
     {
@@ -54,6 +57,7 @@ public unsafe void PreAllocatedOverlapped_NonBlittableTypeAsPinData_Throws()
     }
 
     [Fact]
+    [PlatformSpecific(TestPlatforms.Windows)] // PreAllocatedOverlapped is not supported on non-Windows
     public unsafe void PreAllocatedOverlapped_BlittableTypeAsPinData_DoesNotThrow()
     {
         using (new PreAllocatedOverlapped((_, __, ___) => { }, new object(), new BlittableType() { i = 42 })) { }
@@ -61,6 +65,7 @@ public unsafe void PreAllocatedOverlapped_BlittableTypeAsPinData_DoesNotThrow()
     }
 
     [Fact]
+    [PlatformSpecific(TestPlatforms.Windows)] // PreAllocatedOverlapped is not supported on non-Windows
     public unsafe void PreAllocatedOverlapped_ObjectArrayAsPinData_DoesNotThrow()
     {
         var array = new object[]
@@ -74,6 +79,7 @@ public unsafe void PreAllocatedOverlapped_ObjectArrayAsPinData_DoesNotThrow()
     }
 
     [Fact]
+    [PlatformSpecific(TestPlatforms.Windows)] // PreAllocatedOverlapped is not supported on non-Windows
     [ActiveIssue("https://github.com/mono/mono/issues/15313", TestRuntimes.Mono)]
     public unsafe void PreAllocatedOverlapped_ObjectArrayWithNonBlittableTypeAsPinData_Throws()
     {
@@ -92,6 +98,7 @@ public unsafe void PreAllocatedOverlapped_ObjectArrayWithNonBlittableTypeAsPinDa
     }
 
     [Fact]
+    [PlatformSpecific(TestPlatforms.Windows)] // PreAllocatedOverlapped is not supported on non-Windows
     public unsafe void PreAllocatedOverlapped_ReturnedNativeOverlapped_InternalLowAndInternalHighSetToZero()
     {
         using (new PreAllocatedOverlapped((_, __, ___) => { }, new object(), new byte[256])) { }
@@ -99,6 +106,7 @@ public unsafe void PreAllocatedOverlapped_ReturnedNativeOverlapped_InternalLowAn
     }
 
     [Fact]
+    [PlatformSpecific(TestPlatforms.Windows)] // PreAllocatedOverlapped is not supported on non-Windows
     public unsafe void PreAllocatedOverlapped_ReturnedNativeOverlapped_OffsetLowAndOffsetHighSetToZero()
     {
         using (new PreAllocatedOverlapped((_, __, ___) => { }, new object(), new byte[256])) { }