Galio may play as a tank/Support/mage whatever, but his strong point is still initiating with his AoE taunt. Galio is above all things still an Initiator and for an Initiator to lose his biggest tool to Initiate, for Galio being his Taunt, is incredibly stupid even for Riot's recent balance decisions.
His dash is really easy to dodge despite the fat hitbox because he pulls himself backwards by a bit while looking at the direction he will dash, making it VERY telegraphed, and his ult is slow enough to easily flash out of it even under high pressure. His taunt is the only reliable tool for him to set up anything, and now you can't flash into enemies with that taunt?
Considering Galio has absolutely no slows in his kit means his Taunt will only ever hit if either Galio gets help by another initiator, a peel or successfully lands his E dash, which is, as mentioned before, really damn easy to dodge.
An Initiator who needs an initiation to get going is hypocritical. Imagine making an Assassin who needs damage from allies to work or an ADC who's auto attacks cannot kill an enemy under any circumstances. There is absolutely no non-biased way to defend this nerf, this was pure hypocrisy to target Galio and reduce him into non-existence. They did the very same thing with Rengar and LeBlanc when they had their reworks, and they nerfed them so often and with such stupid ways that they had to revert them both within the same pre-season.
So not only are they being really, REALLY obvious about hating Galio but they are also repeating past mistakes that practically lynched their PR publically and had them launching full reverts.