I'm not really worried about Kitty being dead or something because you're correct, familiars are just spells. However, Kitty also seems to be a special case of a familiar which is constantly shifting, which is something we never encountered before, so I'm kind of worried Kitty simply won't come back the same. We don't know what the shifting barrier does. Also, they didn't say Kitty disappeared or anything — they just stated everyone is safe except for Kitty, which might as well mean something went wrong.