LambdaMOO is a fully programmable text-based virtual reality.

Here's the LambdaMOO programmer's manual: https://hayseed.net/MOO/manuals/ProgrammersManual.html

Webpage: https://www.moo.mud.org/

Other resources: https://github.com/johndoe771/lambda-moo-programming

And for LambdaMOO itself telnet to lambda.moo.mud.org 8888.

I guess? I logged into it and there was some basic world that must have been out of the box demo data, and it seemed like basically a REPL where you could create objects like rooms, and verbs, and a verb could move you to a different room; so like a REPL with state like a shell's pwd and environment. I would just carve off rooms, I think I made some chest-like things. You'd write tons of text for different verbs people might use.

I can't remember any of the code, at all, now though. Just remember constructing "The Ethereal Plane" with all sorts of cloud metaphors for computing back before that was cool~

I spent many many hours playing/coding/wizard-ing with several Star Trek themed MOOs and an original space themed version. I thoroughly enjoyed the language and environment. MOO is really where I initially grasped and thrived with OOP.