I haven't played a MUD in the last 25 years, but I think I would enjoy playing this one if I had time. In some ways MUDs were much more fun than modern games. I wonder what codebase it derives from.
out of the all the MUDs from that era I think the most noteworthy was Assault, which was a Merc derivative mod with a top down map for real time strategy base defense, active around 2006 then disappeared into obscurity, and very wild in a pre-Dwarf Fortress era
As for the JavaScript client, it appears to be proprietary.
[1] https://t2tmud.org/boards/news/1402345353.php
Heavily modified. Language is C like.
Source code has been leaked in the past.