User blog comment:Reilock/Season 4: History/Lore of the League: Thoughts/@comment-5035765-20131108054701

not all the skins are canon, just SOME of them... there are ones that are way un-canon and impossible...

regarding the ones from the future (BC and PF), well, some skins are obviously from the past (Pre-void Kass for one), so it may not be that much of a stretch for them to be able to pull some from the future