MUDs are still kicking around and if you'd like to play in one you'll find a few solid communities with good engagement that would love to include you. They're in an odd spot though since MUDs existed in two sorts of categories 1. I really want to adventure and text is the only way to do that (these sorts died off to MMOs) and 2. I want to roleplay - those survive but the mechanical portions of those games are becoming more and more irrelevant as new games are constantly supplying better systems and mechanisms than MUDs were able to.

This leaves MUDs in a weird place since things like MUSHes generally have most of the roleplaying tools without as much weird code for attacks and PVP interactions being involved so I've seen those thrive more recently.