Are there any MUDs still running, that you would consider to have really game changing mechanics?

I’m doing research while building server framework from scratch, as a pet learning project. I’m familiar with a lot of the basic functionality from playing MUDs a lot as a teen. But admittedly it’s been a good decade since I’ve touched one, and I’d like to find some interesting ones that have pushed boundaries.


EDIT: If anyone is at all interested in following the progress of the project, my github is linked in my profile.