diff --git a/src/xunit.netcore.extensions/Attributes/ActiveIssueAttribute.cs b/src/xunit.netcore.extensions/Attributes/ActiveIssueAttribute.cs
index 9753ab544a..f3267b0ca8 100644
--- a/src/xunit.netcore.extensions/Attributes/ActiveIssueAttribute.cs
+++ b/src/xunit.netcore.extensions/Attributes/ActiveIssueAttribute.cs
@@ -16,9 +16,5 @@ public class ActiveIssueAttribute : Attribute, ITraitAttribute
     {
         public ActiveIssueAttribute(int issueNumber, TestPlatforms platforms = TestPlatforms.Any) { }
         public ActiveIssueAttribute(string issue, TestPlatforms platforms = TestPlatforms.Any) { }
-
-        // TODO #999: Remove PlatformID when all uses have transitioned to TestPlatforms.
-        public ActiveIssueAttribute(int issueNumber, PlatformID platforms) { }
-        public ActiveIssueAttribute(string issue, PlatformID platforms) { }
     }
 }
diff --git a/src/xunit.netcore.extensions/Attributes/PlatformSpecificAttribute.cs b/src/xunit.netcore.extensions/Attributes/PlatformSpecificAttribute.cs
index e1ff21c6bd..5053288481 100644
--- a/src/xunit.netcore.extensions/Attributes/PlatformSpecificAttribute.cs
+++ b/src/xunit.netcore.extensions/Attributes/PlatformSpecificAttribute.cs
@@ -15,8 +15,5 @@ namespace Xunit
     public class PlatformSpecificAttribute : Attribute, ITraitAttribute
     {
         public PlatformSpecificAttribute(TestPlatforms platforms) { }
-
-        // TODO #999: Remove PlatformID when all uses have transitioned to TestPlatforms.
-        public PlatformSpecificAttribute(PlatformID platform) { }
     }
 }
diff --git a/src/xunit.netcore.extensions/PlatformID.cs b/src/xunit.netcore.extensions/PlatformID.cs
deleted file mode 100644
index 8d63e1e357..0000000000
--- a/src/xunit.netcore.extensions/PlatformID.cs
+++ /dev/null
@@ -1,22 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-// See the LICENSE file in the project root for more information.
-
-using System;
-using Xunit.Sdk;
-
-namespace Xunit
-{
-    // TODO #999: Remove PlatformID when all uses have transitioned to TestPlatforms.
-    [Flags]
-    public enum PlatformID
-    {
-        Windows = 1,
-        Linux = 2,
-        OSX = 4,
-        FreeBSD = 8,
-        NetBSD = 16,
-        AnyUnix = FreeBSD | Linux | NetBSD | OSX,
-        Any = ~0
-    }
-}
diff --git a/src/xunit.netcore.extensions/xunit.netcore.extensions.csproj b/src/xunit.netcore.extensions/xunit.netcore.extensions.csproj
index 1068265b27..fac061c83f 100644
--- a/src/xunit.netcore.extensions/xunit.netcore.extensions.csproj
+++ b/src/xunit.netcore.extensions/xunit.netcore.extensions.csproj
@@ -28,7 +28,6 @@
     <Compile Include="Discoverers\OuterLoopTestsDiscoverer.cs" />
     <Compile Include="Discoverers\PlatformSpecificDiscoverer.cs" />
     <Compile Include="Discoverers\SkipOnTargetFrameworkDiscoverer.cs" />
-    <Compile Include="PlatformID.cs" />
     <Compile Include="SkippedTestCase.cs" />
     <Compile Include="TargetFrameworkMonikers.cs" />
     <Compile Include="TestPlatforms.cs" />