Since we just had the 2nd instance where someone laid a massive (6-digit) amount of slabs in the nether to modify spawning behavior, I have now blocked all slabs from being placed in the nether.
For those who do not know, slabs prevent mobs from spawning. But the server will still try to spawn them and just cancel the spawning once it detects that there is a slab on the location. So it will try again and again at very high speed, which lags the server massively. So please don’t do it over large surfaces. In any world.