Only if it's still attached, can doctors and surgeons reattach necrotic limbs and be successful?
Injuries like losing limbs or horns in this case need powers on god like levels or certain potions.
Whether it's a fantasy or not.
Unless it's a super advance sci-fi story where you literally grow new parts or bodies.
Considering what's mentioned in the LN, the shame of losing your horns cannot be undone without an elixer grade potion.
Thus living the rest of your life in shame.
So my best guess from this is, once the horn is removed from certain races, it's a permanent as there is no possibility of reattaching said parts.
I mean, broken bones heal, but as with my previous statement, there'd probably be a very limited timeline if it's at all possible.
That being said, I could still be wrong or have missed a crucial peice of information.
I do know, that later
she gets her horns back when MC figures out how to make elixirs, but a key ingredient requires hurting a future harem member to get the materials and he refuses to do it more then once even, though I'm pretty sure she offers to give more, but he still refuses