I remember from the finale of The Last Wish, that Geralt figures out/guesses that Yennefer was once a hunchback, due to the slightly unnatural stance she had (presumably something like muscle memory, and even then it took even a Witcher long to really notice it, and it felt more like guesswork on his part) and I'm sure that there isn't a single witch in the books that actually uses illusion magic to permanently alter their appearance (while they can appear like different people altogether, that would take quite a bit of energy to maintain for extended periods of time, from my understanding. And it wouldn't fool other witches, not unless it's a very high level spell, which would be even harder to maintain and there aren't that many people that can pull those off to begin with)