Wikipedia's historical list of MUDs ( https://en.wikipedia.org/wiki/List_of_MUDs ) suggests that MUD1 (1978, homepage: https://www.british-legends.com/CMS/ ) and MUD2 (1985, homepage: https://www.mud2.com/CMS/index.php ) might still be active. From what I can tell their codebases are pretty light on substantial updates in recent years, but then again Nethack was at point considered abandoned for about a decade.