[Arrei]

For the despawning cap, that's a separate plugin that forces mobs to despawn if there are too many in a specific chunk. There are separate caps for hostiles and passives, so that monsters won't cause animals and villagers to despawn and vice versa. But if you, say, left a chicken or villager auto-breeder on for too long, they'd start to despawn at whatever location they're congregating at, making it so no further gains are made once they hit the cap of 40 mobs. If you also made the poor decision of keeping these mobs in the same chunk as other passives, they too would disappear randomly. You could do things like create redstone mechanisms that ferry additional spawns elsewhere once a cap is reached, but there is also the separate issue where the staff reserves the right to trim down any farms that grow too large for one area. This is rare, but if you, say, decided to keep 100 of each animal in the middle of a town with other players' animals and item frames and so forth, that's a culling.

I am actually unsure what governs iron golems, as they don't cause villagers to despawn (as evidenced by four-pod iron towers working without issue) but they don't seem to count as monsters either.

However, there is a separate issue relating to the mob cap from vanilla/Spigot outlined in the previous post. That cap is per world instead of per chunk, so even though a farm won't allow more than 40 skeletons or something to stand in a kill chamber, they will continue to occupy their spots in the global spawn cap if not killed, which is why we forbid simply standing around in a farm not killing your mobs.