Don't know if it's intentional or was a patched bug, but in 1.9.3 you could anvil say, diamond helmet+prot 4 with a diamond leggings+unbreaking 3 to get a diamond helmet+prot4+unbreaking 3.

Was this removed?

Addendum: If it was intentionally removed, it would be nice to have this as an optional feature 👍

Yeah there was a bug in the Tome enchanting handler that allowed for merging of anything. I suppose I can put something back in it's place, but it will likely be more expensive than it was.

Reimplemented as a config option.