There is an ongoing project doing this (that launches this weekend I believe) to revive a MUD called Shadows of Isildur. I think a lot of effort has been put into the new changes to the codebase to bring QoL features that a modern audience would expect while keeping the bones of the game the same. I'm not certain how it will fare as, to be honest, MUDs take an inordinate amount of time to really groove into and immerse yourself in - but I am excited to see efforts like this invested on as we older MUDers find ourselves finally reaching the point of having free time again.