Champion Mastery Taunt: Legit Strat or Bannable?

Assuming you just mean using ctrl-6 to show your mastery emote, it's definitely not bannable. BM at worst, but it's a built in game feature and showing it off is basically the whole point of it.

(If on the other hand theres some sort of glitchy bug thing where you if emote/taunt and it lets you recall faster or hides skill shots or something along those lines, that might be getting into bannable bug abuse territory, but I don't think that's a thing.)