C# Async/Await networking server
I see two major issues when you want to use this code for a MMORPG game.

Exception Handling

Make your code robust against connection hick-ups (when connection goes down for a fraction). It's up to be the client to reconnect asap on connection loss (out of scope of your code). The server should dispose client connections on error in order to clean up resources. 

Resource Management

The server does not care about the number of clients connected, nor does it own the life time scope of these connections.

I would expect the code below to close the connections.


public void Stop()
{
    _listener.Stop();
    // _connectedClients.ForEach(Disconnect); // cleanup resources
}