I was too young for the MUD heyday, but I’ve thoroughly enjoyed the community-led rebirth of EverQuest, an early 3D MMO that was heavily MUD-inspired. There’s a free server run by an independent team which seeks to replicate the experience of the game as it existed in 1999. It’s become quite popular, with a regular player count of 1000+: https://www.project1999.com
Personally I don't have the time, energy or interest to play an MMO with the kind of time sinks that Everquest had and that Pantheon plans to repeat. But if it's an itch you want to scratch then look for that game.