What was the point of the Master Yi nerfs?

Neither nerf was successful in addressing Master Yi's issue of being frustrating to play against. He still has a 40% ban rate. That's because that issue arises from the itemization issues that cause him to snowball too hard, which is being addressed next patch.

All the nerfs have done are cause problems. Why nerf his kit the patch before you were going to actually fix him?

The ult nerf didn't even target the itemization/snowballing issue at all, it just served to decimate his identity and his kit that had been stable for 3 seasons. Those 3 seconds that were shaved off were an essential part of being able to gank, catch people out, and team fight.

In a gank, he would have ~5 seconds left on his ult after alpha striking (Which takes off ~1 second from his ult by itself) which, as a jungler with no cc, was necessary for sticking and tower diving. Now that's 2 - 3 seconds left on highlander, that's not enough to do those tower dives.

In a team fight, Master Yi needs to use ~2-3 seconds of highlander for flanking/repositioning/gap closing, he loses 1 second during alpha strike, and then he has 3 - 4 seconds left to try to get a reset and will most likely take 1 cc ability or an exhaust. In this meta, he's not bursting down a tank and ADCs have far more mobility and survivability than they used to due to essence reaver and steraks, so they likely have a gap closer up or on a very short CD and possibly steraks to survive burst. So team fighting looks like a crap shoot now.

That means his primary win condition is just to continue to snowball, abuse current itemization, and perma split push like a Tryndamere. "Just snowball and split push" isn't a reliable win condition in higher elo. Next patch, that snowballing issue will finally be addressed, and he'll stay nerfed just like Sejuani was because of Cinderhulk.

As for the E nerf, it's less significant but it didn't make him less snowbally or build differently, it just made attack speed items like zeal awkward and slapped him on the wrist for building steraks.

tl;dr: The highlander nerf makes team fighting even worse (As a champion notorious for being a terrible team fighter anyway unless snowballed. Which again, is his issue and why he's been better in team fights lately. Which this nerf doesn't address at all), makes split pushing his main win condition which isn't that viable in higher elo, and makes all ganks/tower dives more difficult to pull off.

Credit to Ichigo, Master tier former Yi main (He's just going to play meta junglers with sunfire and IBG for freelo and maybe teemo/Yasuo jungle for fun now). I took a bunch of ideas from his post on what the ult change means.