Does tenacity reduce knock up airborn time?

Not a chance. If it did, it would hurt Yasuo...and we can't have that.

From the wiki:

Tenacity Main article: Crowd control Reductions For the retired defensive mastery, see Tenacity (mastery). Tenacity reduces the duration of all crowd control effects, except for Suppression icon Suppression, Stasis icon Stasis and Airborne icon Airborne effects such as Knockbacks, Knockups and Pulls.

It doesn't affect the duration of knock ups, displacements or suppression.

Nope. We still don't have displacement tenacity.

No. Generally, it applies to anything that isn't a displacement or a suppression or stasis. Champs with suppression include: Tahm Kench, Malzahar, Urgot, Skarner, and WW. Champs with stasis include: Anivia, Lissandra, Bard, Kalista, Zilean, and Aatrox. Zhonya's also causes stasis.

The only way to clear suppression or stasis is by using a QSS item. There's tons of other stuff that tenacity applies to, though.