User blog comment:Paul.rea/Teen Wolf News 041316/@comment-2020973-20160414140247/@comment-26879545-20160418172405

Her departure wasn't vague. The entire season was building up the fact that her Fox Spirit was going rogue and she needed to learn how to control it. One of the first things we see in s5B is her, going to the skinwalkers, in order to learn how to control her Fox. In that same episode, Noshiko hints that it could take centuries for her to learn control, or she might have to become a Skinwalker herself.

She gave Scott her tail for safe keeping. It's similar to when soldiers go away to war and they leave their loved one a "token" which symbolises their promise to return. As of s5B's finale, her story is over. There's nothing more for her to do on the show, since she's going to be training for many, many, many years.

*  "I mean the poor girl had to abandon her family and friends including Scott whom she has spent the last three seasons romancing. That is not a good ending for Kira. She deserved something better then being written off. Hopefully Arden will one day come back to give Kira a proper send off."

She also, literally, told Scott she was doing it, not for him or her parents, but for herself. She left on her own free will in order to recieve the training necessary to control the Fox Spirit. As far as most shows go, that is a "true sendoff."