Zac has a 60% ban rate in Plat +

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You already have a metric in front of you: high pick+ban rates have historically always correlated with champion strength. There is no reason to believe this will change.

High pick/ban rates have a huge impact on a champion's ability to win a game. Consider a champion like galio with a 1% pick/ban rate. His winrate are heavily influenced by the .1% of the community that play him the best into team compositions which fit his kit the best. Now consider a champion with a pick rate of 30%. Mathematically, no more than 3% of the games he is played in are done by the .1% of the community who plays the champion the best. The other 97% of the games represent the champion's performance in the hands of an inferior player.

Now add in a high ban rate on top of that. Champions with high ban rates are generally only allowed through when the other team knows they will be picking a strong counter matchup. Therefore, not only does the winrate reflect weaker players playing said champion, they reflect the champion into far worse matchups. A low winrate in the setting of a high pick+ban rate does not reflect a weak champion, it simply reflects the fact that overpowered champions are forced into suboptimal situations far more than balanced champions.

Zac has gotten much stronger over the last few patches and yet his winrate have not moved nearly as much. This shows that something else is artificially suppressing his winrate (his insanely high pick+ban rates right now).

Your whole argument stems around that high ban rates influence win rates because you would only see the champions played against champs that counter them if they weren't banned. But there is no evidence to show that is true. If you look at Zed, you see he plays about the same overall games against champs that beat him vs champs that he counters EXCEPT there are far more champions that beat him than he counters.

Katarina vs Zed was a matchup Zed used to be able to win or at least go even with. Looking at the stats now, there is no doubt Kat has a large advantage vs Zed and outclasses him in every aspect. Zed loses to so many champions now mid because he lacks the killing power he once had. Once again, the only reason Zed is highly banned is because of the QSS changes. He's an annoyance to ADCs but he is not at all powerful. High level players (Like Bjergsen) have stated Zed is insanely weak right now. There is no metrics that suggest otherwise.