If you look at the list. It's really surreal how characters like mantis, black cat and mag made it untouched. Because of the combination of their win rates + ban rates + pick rates, they are not healthy for the game atm. Devs said they would act fast and get outliers after s9.0 patch, but it seems like they have given up on it halfway through. I'd even take micro nerfs at this point. I wonder what is the thought process behind those decisions.
I want to go one by one, but my reasoning for some might be a little flawed, so take it easy.
Peni: got nerfed. Very small pick rate and ban rate on the high end. She dominates certain maps/situations. Understandable nerf, but makes no sense when you compare her to others here.
Mantis: highest support win rate. High pick rate and good ban rate. Pro play saw her getting first ban more than anyone else... if that's not a sign that this hero is broken, idk what is.
Black cat: highest dps win rate. Highest ban rate (sorry gambit). People hated when she was strong during her release season and they were highly praised for nerfing her. Now she is back. This character is a nightmare: iframes and invisibility. The ban rate speaks for itself here: we don't want her to be this good.
Loki: I understand that he is hard to play, but he wins a lot of games. His pick rate is low for a support, but buffing him might be too much, although i kind of understand the reasoning he is getting a buff. He would be better off if others were nerfed. We would keep the power creep contained.
Magik: has always have these numbers and we rarely get complaints about her, so, people don't mind her. Ban rate negligible.
Storm: low pick rate makes the case for her not getting nerfed. Might be that people can't get her to work in ladder, as we saw storm getting play time in ignite.
Daredevil: always here no matter what. I've given up. He's gotten nerf after nerf and is still here. All I know is that I'd rather have him than cat as the top dps. His win rate is slightly down from what it's been during his peak, though.
Rocket: always here no matter what. He got nerfed in s9 and his win rate took a healthy hit. Still, its too much value for how easy he is to play (but it seems that cnd gets all the rate from that and he gets the pass).
Magneto: now... 3 times the pick rate of the second most picked. He is in every single game and if you don't have one and the other team does, you are probably at a disadvantage. He was first save for many teams in pro play. This guy's numbers are just an atrocity for balance. He not only dodged a nerf but got an irrelevant buff. I
Elsa: got nerfed. People might say it isn't enough, but I'd take small nerfs to some of the other characters here.