So I've just build an xp farm at zombie spawner on my server. The problem is, the zombies start taking damage out of nowhere and dying once they stack up to about 25 in the killing spot. Is that some new mob cap or some kind of server issue?
As Geobell said, that's the maxEntityCramming gamerule, which was added in the 1.11 snapshots. Provided you are an "operator", you can use "/gamerule maxEntityCramming 0" or such to disable it, or just set it to a high number. When a mob is pushing/touching more than this amount of other mobs, it will take damage, and as all the mobs are touching each other, they will just all die eventually if there's enough of them in the same space.