It's just you. Higher elo doesn't make people less intelligent.
Kat is broken because of her passive so she thrives in low cc metas with less tanky champions and metas where her counters aren't played. Kat was generally considered a weaker mid before Kassadin, LeBlanc, and Talon lost their silences. Annie, Cho'Gath, and Diana are all very good meta options against her in mid lane because they can win lane and deal with her later on. Annie and Cho'Gath can kill her during their instant cc and Diana can stop her ult and force her to E or Zhonya out of a fight.
Yasuo nerfs seem out of place because he isn't in a very good place right now in terms of overall power. He's a very strong duelist, skirmisher, and split pusher, but he's not very good at much else. Yasuo mostly relies on his base movement speed early to be able to dodge and outplay his opponents before getting shiv/boots and levels in E. The nerf to his early game (via base movement speed) just makes that Godsuo splitpush dream so much harder to get to and that's the only good thing about him.
The difference in the two cases is you can play against Katarina to some extent (though nothing really stops the snowball once it's rolling in a fight) while Yasuo is taking a hit to one of the few tools he can use to be strong. Options vs no options.
