It does break the game.
If Clear quest zombies don't spawn because the server limit prevents it, the quest can't be completed.
As there were other bugs also causing the same result this variant appears to not be common knowledge.
I don't think it needs to and maybe isn't already. As long as the game reserves one or more zombies per player (or despawns zombies from other players with surplus zombies if one player would be without one) then every player with a clear quest has at least one (or more) zombies in the building to kill.
The game already had similar code to ensure something like that, maybe only on horde night. It was 64 max zombies for all and 8 for each player max, because 64 zombies / 8 players = 8 zombies. Not sure if this is still the case.
That solution is rather trivial but rather unflexible. I think one reason for the event system they wanted to implement was to get a more dynamic system
