Wither rose skull farm

A wither skeleton farm built straight onto a nether fortress, with no redstone in the spawning part at all. You slab or carpet over the whole fortress so nothing can spawn there, leave one platform of nether brick with carpet on it as the only spawnable surface, and wither roses on that platform kill everything that lands. Hoppers underneath carry the skulls, bones and coal away. It is best built in a soul sand valley or a warped forest, where the only other spawns are skeletons or endermen rather than the piglin and hoglin packs that swallow the mob cap. The spawnproofing is the whole job, the build itself is small.

Put nether brick under the carpets. Wither skeleton pack spawning needs it, which the creator had to add as a pinned note after the video went up.
Turn your render distance down to 2 chunks. It is the most repeated tip on the video and it does more for the rate than anything else you can do.
Spawnproof the entire fortress, not just the platform. Every "it does not work" report traces back to this or to a nearby biome still spawning mobs.
captainferrite recommends walling in the 25x25 platform area before anything else, otherwise blazes and ghasts blow up your work while you build.
Do not carpet the whole floor in wither roses. The skeletons get stuck on them, so REALQwvBY leaves paths between them instead.
TreverBeeftown-sd7di skipped the piston part entirely, let the mobs drop onto hoppers and killed them by hand, and it still worked on a multiplayer server.