It definitely is still stealable. The skill was just changed from being an ultimate to an encounter skill. My "extensive testing" came from me being confused as to how there wasn't a learnable ult in the succubi's repertoire.
Apart from that, "Tira's Embrace" is a powerful spell but not overpoweredly so and fits in well with all the other level 8 stuff that was added into the game. Just leave it as is.