Brown mushroom slime farm

ilmango's swamp slime farm, the one that stopped people digging out slime chunks. Brown mushrooms emit light level 1, which is dark enough for swamp slimes and too bright for everything else, so you carpet the whole spawning platform in mushrooms instead of spawnproofing anything. The platform sits in a swamp between Y50 and Y69, an iron golem in the middle pulls the slimes across it, and powder snow freezes them to death over a single hopper. You AFK on a scaffolding tower about 120 blocks above. Swamp slimes only spawn at night and the spawn chance follows the moon phase, so the output arrives in nightly batches.

Every single block needs a mushroom on it. One missed spawning spot lets a mob in and the iron golem dies, which is what nearly every broken build turns out to be.
Stand on the spawning platform and check you are at Y69 or lower. Above that nothing spawns at all.
ancillarity built it with redstone torches instead of mushrooms, spaced out, and says the rate is fine for a day one farm. ilmango replied that it is a really good idea.
ilmango's own alternative is a checkerboard of magma blocks. They light the surrounding block to 1 and kill the slimes as well, with minecart collection underneath.
juggernaut4799 wears leather boots while building so the powder snow does not catch him, and warns that the last mushroom by the exit can break from outside light as you close the room.
A mangrove swamp works just as well as a normal one.