Minecraft (client) never does that, so you are still connected, but not "in world". I don't know if it's easy to shut down netty thread from MC main thread, but should be relatively easy to do so. (with try-catching everything of course, some mixin might inject into closing process and cause double crash)

It's unlikely I will implement more features myself, but I will accept a PR.

quite possibly i'll be able to make one in a few days