I'm happy I stumbled upon your post! I actually am building a small one of these myself for exactly the same reason! I never really interacted with MUDs back when they were more prevalent, but I've recently checked out a few of them. I wanted to build something like this for a small group of friends that I play TTRPGs with.
There actually are quite a few of these servers still running! You can find a list of some of them
here. I've found that a lot of these are very RPG-focused and play more like text-adventure games than open roleplaying environments, but they all have RP elements to them, some more than others.
If you're looking to build something like this, might I recommend
Evennia. It's a Python framework for building text-based multiplayer spaces and does a lot of the heavy lifting for you. I've been extremely impressed at how quickly I've been able to pick it up and start building things in it. I'm not an expert, as I've only started playing around with it recently, but they have a thriving community of developers and the documentation is great.
You can access these very easily with a client like
Mudlet. This comes preloaded with a few connections to some of the bigger MUDs that are still running, and I've found that a lot of these communities are quite friendly.
If you end up playing around with building something like this I would love to hear more about it!