Rather move mentionable to interface for consistency.

src/Discord.Net.Core/Entities/Users/IThreadUser.cs

@@ -2,7 +2,7 @@ using System;
 
 namespace Discord
 {
-    public interface IThreadUser
+    public interface IThreadUser : IMentionable
     {
         /// <summary>
         ///     Gets the <see cref="IThreadChannel"/> this user is in.
@@ -13,5 +13,8 @@ namespace Discord
         ///     Gets the timestamp for when this user joined this thread.
         /// </summary>
         DateTimeOffset ThreadJoinedAt { get; }
+
+        /// <inheritdoc />
+        string Mention { get; }
     }
 }

src/Discord.Net.Rest/Entities/Users/RestThreadUser.cs

@@ -7,7 +7,7 @@ namespace Discord.Rest
     /// <summary>
     ///     Represents a thread user received over the REST api.
     /// </summary>
-    public class RestThreadUser : RestEntity<ulong>, IThreadUser, IMentionable
+    public class RestThreadUser : RestEntity<ulong>, IThreadUser
     {
         /// <inheritdoc />
         public IThreadChannel Thread { get; }