Add obsolete attribute to DisablePrivateReflectionAttribute

src/libraries/Common/src/System/Obsoletions.cs

@@ -48,5 +48,8 @@ internal static class Obsoletions
 
         internal const string WebRequestMessage = "Use HttpClient instead.";
         internal const string WebRequestDiagId = "SYSLIB0014";
+
+        internal const string DisablePrivateReflectionAttributeMessage = "DisablePrivateReflectionAttribute has no effect in .NET Core and .NET 5.0+ applications.";
+        internal const string DisablePrivateReflectionAttributeDiagId = "SYSLIB0015";
     }
 }

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/DisablePrivateReflectionAttribute.cs

@@ -3,6 +3,7 @@
 
 namespace System.Runtime.CompilerServices
 {
+    [Obsolete(Obsoletions.DisablePrivateReflectionAttributeMessage, DiagnosticId = Obsoletions.DisablePrivateReflectionAttributeDiagId, UrlFormat = Obsoletions.SharedUrlFormat)]
     [AttributeUsage(AttributeTargets.Assembly, AllowMultiple = false, Inherited = false)]
     public sealed class DisablePrivateReflectionAttribute : Attribute
     {

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -9381,6 +9381,7 @@ public DependencyAttribute(string dependentAssemblyArgument, System.Runtime.Comp
         public string DependentAssembly { get { throw null; } }
         public System.Runtime.CompilerServices.LoadHint LoadHint { get { throw null; } }
     }
+    [System.ObsoleteAttribute("DisablePrivateReflectionAttribute has no effect in .NET Core and .NET 5.0+ applications.", DiagnosticId = "SYSLIB0015", UrlFormat = "https://aka.ms/dotnet-warnings/{0}")]
     [System.AttributeUsageAttribute(System.AttributeTargets.Assembly, AllowMultiple=false, Inherited=false)]
     public sealed partial class DisablePrivateReflectionAttribute : System.Attribute
     {