As my slime boots decay to fast I wanted to put "mending" and "unbreakable" on them. But no.
The sling also resists.

Is that the intended way it should work?

What I tried: books in anvil and throwing it into the enchantment table.
The table seems to work partly. I can see enchantment rows turning light but they are not deciphered and clicking on them has no reaction.

No crash! That is good.

No that's not intended behavior, I will fix it soon. Thanks reporting this