Stepping in to clarify here; Masters of Ruin have had a spawnlimit of 1 set since the beginning, so that's why there hasn't been a changelog called out for them. If you're encountering more than one at a time, that would have to be a result of multi-aggroing spawns.
Masters of Death do not have a spawnlimit, and aren't expected to get one, as the mage class that should exist most commonly at high level.