Djlanfull
I have not seen an MMO that's still being played not updated.
Phantasy Star Online: Blue Burst and Universe. (No wonder Sega got massive cheers when they announced PSO2.)
Uh... forgot the name, something like "The World"... but there was one where you'd create these REALLY psychedelic rooms and characters and just show it off/talk to people. It was really nightmare-ish, and had some bugs that were left unatended because they could add to the ******** feel, such as clipping trough walls. (/x/ keeps reviving that game)
Minecraft will be this, sooner that you'd expect. Notch is releasing the source code to modders and going to work on "Scrolls". What faint notion of periodic updates you could imagine coming from Notch and his company will be gone, most definitely.
Hmmm... A bunch of those Text based MMOs might have lost their developers as well but keeping them up is relatively simple.
Most MMO's that aren't updated are taken down based on one thing alone:
No one hosts servers. In the rare, far exceptions that someone does, first, it's going to be a private server thing, second, they will probably alter SOME stuff on the game as well. I know Schthack changed some minor things on his private server, but it was barely an update.
zOMG!, being part of gaia, could very well become the first MMO still endorsed and kept up exclusively by a company that made it that gets absolutely zero modifications in the long run.
On the flipside Valve managed to update Team Fortress Classic recently (a game well over a decade old)
