Knock ups used to be rare/unique

We have 31 champions on each page for "knockup" and "knockback" on the wiki, some of them are on both while others aren't. So that would mean around 50 champs have an airborne Crowd Control which is unaffected by Merc treads (unless it has a stun attached to it). that just further carries into how egregious knockups/airborne has become. http://leagueoflegends.wikia.com/wiki/Category:Knockback_champion http://leagueoflegends.wikia.com/wiki/Category:Knockup_champion

Knock ups used to be rare, and few and far between so it made sense for it to not be affected by tenacity and to be in a realm of it's own in terms of CC. Now we just have so many, they stack (unlike if Xerath/ahri/ and Annie/irelia/fiddle etc stunned at the same time), and they have really wonky interactions that are borderline game breaking at times.

Displacement affects that are unaffected by tenacity need to have some form of rework or addition to Tenacity at this point with just how many are in the game.