Unbreaking and mending can applied to the elytra, but not the curse of binding, which seems to be an oversight.
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Comments 4
Same goes for sword, enchanted with book with curse of binding (anvil) - the end result is without it.
The sword cannot go in an armor slot, so that one is wai.
To me this didn't seem like an oversight, but rather a way to prevent you from getting stuck with an item equipped in survival with the only way of getting rid of it being death, and with either keepInventory or hardcorde mode not even that being an option (elytra cannot be broken unlike armour), meaning you can never again unequip your elytra - whether you'd want to do that to equip a chestplate or to stop gliding in midair.
migrated
Basically MC-107601.