Since the advent of MUD gaming, there has been a wide variety of codebases, servers, and clients, with distinct genealogical lineages, that appeal to quite distinct sectors of nerd-dom, and sometimes with very little overlap in the player base.
The original big-3 "non-Tiny" MUDs were: AberMUD, DikuMUD, and LPMud. These were largely unknown to me as I never played any of them, but they were mostly known as combat-based, player-programmable, and even quite distinctly European, for the most part. I could be wrong.
TinyMUD Classic began at CMU with Jim Aspnes, but quicky a lot of forks developed that resulted in MUCK, MUSH, MOO, FuzzBall and other interesting derivatives. The Tiny* class shared a lot of playerbase, because the appeal was mostly the same; all were user-extensible but only some were programmable; none were combat-based or adversarial. They mostly tended to evolve into social and chat functions, once they were established and built-up.
There are plenty of other MUD types and in the 90s, every budding programmer had ideas and forked a codebase for their own purposes. But it still baffles me, when commenters here and there wax nostalgic about those MUD days, I really have no idea what sort of game they're referring to, if they cannot be more specific.