Aaaand they broke Yasuo
Yasuo had a strong story point in his rivalry with Riven, and it is now gone. Let's just get that out of the way. There's no debating that his hatred of Riven was central to his character and it's now absent. But let's talk about something else for a minute.
Yasuo was Taliyah's master? Look, I like the premise as much as the next guy. I like Yasuo, I like Taliyah, sure. Whatever. There's just one little problem with that: There is nothing to show for this master/student relationship in the gameplay. Hear me out here. The point of a master/student relationship isn't just "Oh hey, this person was this other person's mentor." If they wanted to call him a mentor, that's one thing, but that's not what they did. They very clearly, following Asian martial traditions, made him her master. In "the ways of elemental magic".
This isn't enough. You know why? Masters teach their pupils their way of fighting. Yasuo is a swordsman first and foremost. Does he utilize wind magic? Absolutely. But even that isn't enough to call him Taliyah's master. Nothing about Taliyah's style of fighting matches Yasuo's, except arguably the way they're both meant to be moving around a lot while fighting.
Let me explain why "the ways of elemental magic" doesn't suffice. Quite simply, it's too broad a category. Let's say you're doing a breakdown of real world martial arts, right. So let's start all the way at the top:
Bujutsu. The fighting arts. In theory, one could classify most every traditional martial art under this umbrella. Within that you might divide it between armed combat and unarmed combat. Within armed combat you have kenjutsu, iaijutsu, soujutsu, naginatajutsu, shurikenjutsu, kyuujutsu, etc. Within unarmed, you might divide it between soft and hard styles, or you could jump right to the meat. Kenpou, aikidou (if you want to be technical), juudou, jiujutsu, karate, what have you. Within any one of those, you have your schools (kouryuu in Japanese arts). Kashima Shin-ryuu, Ittou-ryuu, Maniwa Nen-ryuu, Suiou-ryuu, Tatsumi-ryuu... Each school teaches kenjutsu, but each school's style is going to be different.
Now let's look at fantasy. Let's take magic as the example here.
Magic is the blanket term. You might then divide it between, depending on the laws of your universe... Elemental magic, dark magic, primal magic, healing magic, etc. Now let's look at elemental magic. Within that group you have fire, water, earth, wind, maybe ice, thunder, wood, depends on which elements your universe classifies. But within that there's another step still. Let's look at wind because it's on topic. The style in which a mage wields wind is likely to vary from mage to mage, so long as we're comparing things to Asian martial arts. Wind is just an element, not a method. It's the method of using wind that defines the mage. So you might have, just as a rough division, mages who focus on the cutting power of wind, the blasting power of wind, the agile nature of wind, the chill of wind... There are various different ways to use it, and logically speaking, a given "art" would focus more on one of those ways than others.
Yasuo's use of wind is clearly force-related. He uses wind primarily to transfer the force of his attacks across distance, lift opponents into the air, push himself forward, and repel projectiles. Yet evidently, the new lore states he taught Taliyah the art of "wind cutting rock". So Taliyah, an earth mage... was trained by a man who uses wind magic primarily as a force, rather than an unseen blade?
At best this is like saying Yasuo, who studies Ittou-ryuu, was the master of Taliyah, who uses Tatsumi-ryuu. More accurately, this is like saying Yasuo, who studies kenjutsu, was the master of Taliyah, who employs soujutsu.
Think about it. It would be one thing if Taliyah utilized Yasuo's skill set in any way, but what does she do that in any way reflects Yasuo's style of fighting?
Q: Yasuo throws a whirlwind from his sword. Taliyah launches a rock (or several). The only similarity is that they're skillshots, and that's entirely mechanical. W: Yasuo throws up a wall of wind. Taliyah launches an enemy with a rising piece of rock. E: Yasuo lunges at and then past an enemy. Taliyah lays a field of rock mines. R: Yasuo leaps into the air and, presumably, uses wind to to slice through a group of nearby enemies all at once. Taliyah raises up a wall of rocks. Passive: Yasuo generates a shield, one assumes of compressed wind. Taliyah rides the earth.
Any way you look at it, in the realm of martial arts, Yasuo is using a sword and Taliyah a spear. Their styles are nothing alike. Tell me, again, how Yasuo is her master? Swordsmen will absolutely pick up principles from various schools sometimes, and mixed martial arts is all about adapting principles from different arts... but I don't see any of this in Taliyah's style of fighting.
This is a time-honored principle in Asian nations, Riot. If you're going to make use of it, that's great, and I actually really like the idea of Yasuo training Taliyah. But I really wish you'd at least alter one of them mechanically to reflect the changes before you do. At this point, it feels like you're just waving around foreign principles you don't fully understand, which comes across as a little insensitive.