Hextech Gravity Boots to reduce knock-up duration and knock-back distance by X%

Nay, but I think your idea is interesting and could lead to something that could be "Yay"

Just make Tenacity effect knockups and knockbacks (at a 50% effectiveness for knockbacks) already and cap Tenacity to 50% with all sources being additive.

Introducing an item to reduce the cc of cc that is not meant to be reduced it dumb The base time for those cc abilities should be reduced in general, rather than adding counterplay

Could be a middle man that offers defense in both armor and magic resistance in exchange for a situational unique ability.

Yay.