Minecraft wiki entry for Best_enchantments_guide, associated text: Protection, Projectile Protection, Blast Protection, and Fire Protection cannot be combined.
Sharpness, Smite, Bane of Arthropods, and Cleaving [upcoming: JE Combat Tests] cannot be combined.
Silk Touch and Fortune cannot be combined.
Infinity and Mending cannot be combined.
Piercing and Multishot cannot be combined.
Both Loyalty and Channeling cannot be combined with Riptide.
Frost Walker and Depth Strider cannot be combined.