Wither Skeleton Farms
Wither Skeleton farms are slightly modified on VanillaX for two reasons.
Some designs do not work on VanillaX due to them relying on mobcap manipulation, which is not possible on VanillaX due to the reduced local mobcap. This includes designs like Ianxofour's instant Wither Skeleton farm.
All Wither Skeleton farms must have their AFK spot 30 blocks away from the farm itself, this is due to Wither Skeleton pack-spawning AI being disabled if the player is too close to the Wither Skeletons.
Semi-Automatic Wither Skeleton Farm
EASIEST Auto Wither Skeleton Farm Tutorial! 130+ Skulls/hr! | Best Early Game Minecraft 1.20 Farms - YouTube
Note, you do not need to excessively spawnproof an area for this farm due to the 48 block spawn radius on VanillaX. You can use a mod like MiniHud to see the exact amount you need to spawnproof.
Advanced Wither Skeleton Farm
This farm should only be attempted if you have extensive experience with farms or designing farms.
My 3 Levels of Wither Skeleton Farm in Minecraft (1.16-1.19) - YouTube
Showcase by Stromne
It is recommended to use a mix of design 2 and 3 from this video.
Note that portal slicing is not possible on VanillaX, however setting up a system where pistons push Wither Skeletons into a Nether Portal is still an option.
While this is not an exact design, you should be able to figure out how to make one if you know what you're doing.