(PSA) About Zac's E
Zac's E, Elastic Slingshot is defined as a Dash that only ignores terrain. Despite its a appearance league is 2 dimensional. There is no height dimension. You only move across the map dimensions of Width and Length. The appearance of champions flying is an illusion made through animation.
What that means is that CC Abilities fired down Zac's trajectory while he's in his flight path to your position WILL ground him wherever he is struck. It's the same as Tristana's W.
Your counter argument is probably gonna be "'But he dives in at such long ranges and odd angles I can't always be ready for him."
Yes, his E is quite versatile...it's his signature move. However you HAVE to be MENTALLY PREPARED and spiritually steel yourself against his specific brand of ganking bullshit.
For top and bot, you have to fight further up from river and ward deeper.
But most importantly, if you have CC you can fire it into him before he arrives to cancel his engage.