Playing around with statistics trackers like blitz and tracker.gg it looks like jett, reyna, sage, omen, raze are all picked more frequently for the majority of ranks.

Three duelists and a sentinel are picked more commonly than killjoy, so I don't really understand why she's being nerfed. In some ranks Jett and Reyna are picked so often that you're just as likely to see a Jett and a Reyna on a team as you are to see just one killjoy. You're more likely to see a sage than a killjoy (in the majority of games for your specific rank that might not be true).

The Jett 'nerf' we had awhile back wasn't really a nerf at all. If anything she's getting picked more after it. I am certainly bias here, being a kj main who has played her since her release. But when I look at these statistics sentinels all sit somewhere in the middle, not picked the most, not picked the least. chamber and cypher near the bottom, kj and sage near the top, all of them not at the extreme ends. They all still get played.

Then you have duelists, which has Jett the top picked character in the majority of games since release, and Reyna right behind her. Then you have neon and Yoru near the bottom with only a handful of people who play them, and those people mostly play those agents.

You have sentinels in this nice all near the middle balanced position, then you have duelists which have the most played and least played agents but sentinels are changed so much more often, and very often they become weaker over time.

I guess, why target sentinels when they are arguably the most balanced, they all are valid agents, none of them dominating the game. Why killjoy when she's not the most picked?