Added missing implementations

src/Discord.Net.Rest/Entities/Channels/RestTextChannel.cs

@@ -75,6 +75,8 @@ namespace Discord.Rest
 
         private string DebuggerDisplay => $"{Name} ({Id}, Text)";
 
+        bool ITextChannel.IsNsfw => ChannelHelper.IsNsfw(this);
+
         //IMessageChannel
         async Task<IMessage> IMessageChannel.GetMessageAsync(ulong id, CacheMode mode, RequestOptions options)
         {

src/Discord.Net.WebSocket/Entities/Channels/SocketTextChannel.cs

@@ -110,6 +110,9 @@ namespace Discord.WebSocket
         }
         
         private string DebuggerDisplay => $"{Name} ({Id}, Text)";
+
+        bool ITextChannel.IsNsfw => ChannelHelper.IsNsfw(this);
+
         internal new SocketTextChannel Clone() => MemberwiseClone() as SocketTextChannel;
 
         //IGuildChannel