The damage mitigated by Yasuo's shield acts differently than all other shields in the game, in that it regards him as not being hit at all, even when crowd controlled. Things like zilean's slow, a slow from randuins omen, or exhaust all apply a slow without dealing health damage, yet will still get the kill from a tower. However a snare from Nautilus auto attack isn't enough to get the kill instead of an execute.
ZWEF1 votesthat's not a bug its part of the game for some shields as far as I know. I've seen people pop actives and still get executed because the active made the damage not effect the hit points.
Illıum1 votes[{quoted}](name=Kylo Shen,realm=NA,application-id=LqLKtMpN,discussion-id=1EzbHGty,comment-id=0000,timestamp=2017-03-17T06:18:38.456+0000)
that's not a bug its part of the game for some shields as far as I know. I've seen people pop actives and still get executed because the active made the damage not effect the hit points.
My friends actually tested this with Face of the mountain and it still awarded a kill. Even if the damage mitigation saved his execute status, he was still hit with CC. During their tests my friends tested things like randuins omen active, zilean slow, and exhaust. All methods of CC that do no damage, they all still award kills from a tower.