Technically MUDs still are a thing, there's a good number still running. Though sounds like you don't really just want a MUD where you want to go out mobbing but a more social thing (but not IRC?).

I suspect MUDs may have a bit of a resurgence soon, I've seen more and more interest in them as of late simply because they seem like an excellent pairing with LLMs which easily consume text. Though I think for your needs someone would need to work on the barrier to entry to getting on a MUD as well as the modern UI/UX expected today.

Having stepped away from MUDs ourselves for years, it's been interesting to see what the community has been cooking up in terms of UI/UX, Evennia, MUDlet, TUIs, etc.