I’ve been chipping away at one on my stream using Haskell. Experimenting with Lua integration, parsing, networking, STM and concurrency… currently hacking on Entity Component Systems and unboxed vectors and all kinds of stuff.
Thing is you can start small. Start with a chat server. Add character registration. Add a mob. Add some commands to interact with the mob. Just keep adding things on.
There’s a community for mud coders too. There are dozens of us.