Tenacity affecting knockups? Yes. Tenacity affecting displacements? No.
At this point, because they're so darned widespread, it should do. But only for direct knockups; it should not affect displacement effects, for a variety of reasons.
A larger and larger portion of the hard crowd control in League these days is displacement effects (including knockups). It's being trickled in more and more often than other forms of actual disables, which disproportionately affects melee champions' ability to function. Their threat range is often significantly smaller than ranged champions', and it's so much easier to get out of that as a result.
Champions with disabling effects in their kit, in the last two years (plus change):
: No displacement/knockup. Slow -> Sleep [E].
: Three effective knockups, (one displacement due to terrain creation): [AA] (Brittle target), [E], [R2]. 2 Slows [Q], [R1].
: One knockup (potentially) [W, Rhaast], otherwise, Slow [W, Kayn/Assassin].
: One knockup [W]; Charm [R].
: No displacement/knockup. Root [E].
: One displacement (secondary targets only) [R]; Slow [W]; Stun [E].
: One knockup [R]; Root [Q]; Slow [E].
: Two displacements [Mounted Q], [R]; Slow [Q].
: Two displacements, (one due to terrain creation) [W] [R]; Slow [E].
: One displacement [R]; Stun [Q]; Slow [R].
: No displacement/knockup. Root [W]; Slow [E].
We've had twelve champions released since the start of 2016. Every single one, bar one, has had some form of crowd control in their kit (one with an excess of it), which is fair.
Literally three quarters of those champions had abilities that were not affected by Tenacity in the slightest.
Many of those that have displacement effects, would be pointless if Tenacity reduced how far enemies moved:
- Camille's ult would not be able to push enemies out of the arena, rendering that part useless.
- Kled's abilities would function mostly the same since they're small displacements (seemingly mostly used to emphasize impact/cause interrupts).
- Taliyah's W -> E combo basically wouldn't function. You could very plausibly get stuck inside her R wall.
- Aurelion Sol's R would be hit hard, as its purpose is to damage, but also to displace enemies to a fixed distance away from him.
I really do believe that they should tone down on the amount of non-reducible crowd control, they've hit a point where it's so prevalent that it's going to be very difficult to change those abilities/effects without throwing so many things into flux.
I'm actually considering putting together a spreadsheet just to see how the distribution of disables has evolved over time... Hmn.
EDIT: I thought something was off, and I was right! I am an idiot and didn't count the goddamned nine champion reworks (eight with crowd control) released in that space of time. The general trend of irreducible CC still mostly holds, though.
: No displacement/knockup. Stun [E].
: One displacement (due to terrain creation) [W]; Slow [E].
: No displacement/knockup. Fear/Slow [E]; Suppression [R].
: Two knockups [E], [R]; Taunt [W].
: One displacement [E]; 2 Slows [Q], [R]; Stun [E]; Suppression/Fear [R].
: No displacement/knockup. Slow / Charm [W].
: One displacement [Passive]; Slow [W]; Root [E].
: No displacement. Stun [E]; Disarm + Slow [R].
These ones would not be anywhere near as adversely affected by CC reduction being applied to knockups, they're mainly glorified stuns. (Urgot's flip would be hilarious, though, you might get stuck inside his model.)