Yasuo is already incredibly toxic to fight as a Mage and he does NOT need more magic resist.
He already hard counters mages with an ability to deny all their burst. He does not need to be buffed towards them.
Edit: just to bring a little clarity to my point, im not talking about ONLY his windwall to a mage. His entire kit is incredibly toxic. He can clear early waves in about 8-10 seconds (which is way faster than anyone else) which will force the wave under tower, and a mage is gaurunteed to lose a lot of cs. He can use his E to dodge all their skillshots (most mages rely on them) and if he slips up once? "Face the Wind!".
mantra E and even janna ulti if you use it right.
has enough mobility on his own dashing through minions and champs and monster camps to disengage/flee/dodge. This champions kit is so overloaded and stupid it is no wonder yas mains are grouped in with riven mains when it comes to toxicity.