There was bugs with it that weren't allowing players to play it correctly. I don't think it's going to be added back. If anything it'll be for awhile.
Since the 1.8 compatibility update, mob arena would freeze at a certain round (I think 17) after spawning the boss and would stop spawning mobs completely so people got stuck in mob arena with nothing to kill and the rounds would not end unless everyone did /ma l or logged out, or a mod manually restarted it. Cyp disabled it entirely because too many people got stuck without knowing how to get out, and no one could figure out how to make it work properly again.
