With 1.16 now giving a proper use for gold, and 3D biomes, this mod sets the biome above the nether's roof to be that of the void preventing any mob from spawning there.
The roof is still safe to travel, but mob farms simply will not function anymore. As of 1.0.0, a gamerule (killOnRoof) has been added to enable void damage on the roof; it is off by default.

