To my knowledge, the Puzzlebox MUCK closed down over a decade ago. Sometime between 2005 and 2010, according to the Wikifur article.
I know that 12Fold replaced it, to some degree, but that server *also* shutdown some time ago and is no longer in existence.
The latest similar iteration might be considered the Postfurry Worlds MUCK - Which is admittedly still in operation. However, while technically functional, I would categorize it as "extremely deserted", as it typically has anywhere from 4-6 people online at any time, and of those people, 5-6 of them are perpetually idle.
So, uh, I don't think it's actually a going thing anymore sadly.