My understanding is that UMoria is an independent, complete remake of the original Moria. NetHack on the other hand is the same project with the same codebase throughout its history. With that in mind, NetHack is a few months older than the Moria still under development today.
Not to say that this is necessarily the right way to look at it. It's not clear-cut, is what I'm saying.
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.
You forgot the forks like Slashem and the like, which were born from Nethack 3.4.3.
You don't have to look very far.
https://en.wikipedia.org/wiki/Moria_(1983_video_game)
> Stable release 5.7.15 / 4 June 2021; 4 years ago
My understanding is that UMoria is an independent, complete remake of the original Moria. NetHack on the other hand is the same project with the same codebase throughout its history. With that in mind, NetHack is a few months older than the Moria still under development today.
Not to say that this is necessarily the right way to look at it. It's not clear-cut, is what I'm saying.