I've been working on a Wither Skeleton Farm in v16.5 and have been having trouble getting them to spawn.
The farm is built inside of a Nether Base, above a lava ocean. I am using full Netherlands Bricks for the floor and walls, and cobblestone slabs for the roof to prevent spawns. I started with 3 blocks of height, but increased to 4 to troubleshoot if that was the problem. The floor is at y47, and my spawning platform is 33x60.
I've slabbed up most of the surrounding area, again to prevent spawns outside of my farm. I've seen some wither skeletons in parts of the Nether base that I haven't slabbed over yet, so I know that they spawn in this area. But so far Endermen are the only mobs spawning inside my farm.
Other things to note: the biome is warped forest. There is a blaze spawner nearby, but it's not active when I'm in my farm. There are a couple of lanterns outside of the entrance, but no lighting at all inside if the farm.