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.