League of Legends: Does entering stealth during the start of the animation of an enemy spell/auto attack cancel them?

Solution 1:

Going into stealth does not cancel any targeted abilities that are already travelling. So if an auto attack is in the air, that auto is going to hit, regardless of whether you stealth or not. The same thing happens with actual abilities as well, including: Brand ult, Janna W, Shaco E, Malphite Q, and others.

If you're looking for something that will outright cancel these types of abilities, either a spell shield (such as Nocturne's W or Sivir's E) will block abilities, and Zhonya's will cancel any auto attacks currently travelling to you.

Stealthing before the auto attack actually goes off will prevent the auto attack from firing, since the enemy cannot target you anymore. The timing for that is very precise though, similar to stutter-stepping/kiting. You can test this with any champion by going into the practice tool and getting the caster minions to aggro to you. Once they start shooting projectiles, walk into a bush. You'll see that if you time it right, they'll wind up their attack and then cancel it once you walk into a bush. Any autos that were already in the air will still hit you though.