In regular gameplay, all four of the Protection enchantments are incompatible with each other. But as of 1.21, with datapacks it is possible to edit exclusivity by way of the exclusive_set folder within tags.

However, it seems to only respect an all or none situation when editing the original exclusive_set/armor.json.

Steps to Reproduce:

1. Load into world with protections_all datapack loaded in. (This datapack simply removes the exclusive_set fields from the four Protection enchantments)
2. Gather four chestplates each with only a single different Protection enchant.
3. Attempt to place them into an anvil, and observe the output. At this stage they can all be combined just fine, potentially allowing all 4 enchantments on one armor piece.
4. Save and exit out of the world, and replace the protections_all datapack with the special_protections_test datapack (this one removes the exclusive set from just the Protection enchantment and also edits the exclusive_set/armor to only have Fire/Blast/Projectile Protections be incompatible with each other)
5. Attempt to place one chestplate with Protection and another chestplate with a specialized Protection (Fire/Blast/Projectile) together in the anvil slots, and observe any preview output.

Expected Result:

Chestplates can combine into a chestplate with Protection and one specialized Protection.

Actual Result:

Chestplates cannot combine despite the Protection enchantment json and the exclusive_set/armor tag being changed to allow Protection and one specialized Protection.