FULL DISCLOSURE: These are products I created and operated by my company. But very relevant.
The term MUD tends to harken back to an earlier time before 'puters had graphical horsepower of any note. But the reality is, online text-based games come in all varieties, and the one's we operate are in a league all their own. More significantly, they are still serious ongoing commercial efforts. If you want to see what a MUD can be when it's been in continuous development, expansion for decades, then check out:
GemStone IV, which began its life as a sequel to GemStone ][ (then called GemStone III just to confuse everyone) first came to existence on the online service GEnie. Eventually it moved to CompuServe, AOL, Prodigy and others. When online services went the way of the doodoo bird, we moved them to the internet machine. DragonRealms is somewhat younger than GemStone, but same sort of history. I began work on it shortly before forming my company Simutronics, something like 27 years ago.
Despite having worked on lots of other types of games, such as mobile titles, and working on other PC/Mac/Linux games of a much more graphical variety now... these text based games remain the corner stone of Simutronics.