There has not been such a drop in the number of matches won for a really long time.
Riot knew that they had gone overboard with the buffs, as a result of which Master Yi started winning the most games in all of LoL. Winrate hit 55%, and Riot had to make additional patches to the servers for patch 12.5.
The amendments focused only on Master. It was decided to nerf his “Q” as well as “E” and “R”. The nerfs look like this:
We didn’t have to wait long for the effects
It’s rare for nerfs to be so severe on character stats. Master Yi is dipping downhill at this point. At a level above Diamond, his winrate, which was previously 55%, is now around 40%.
Such harsh treatment of “OP” characters is the norm in Riot. According to the developers, it is better to slowly buff a character that is too weak than nerfing a character that is too strong. So Master changed from the best hero to the weakest one.
In the overall ranking, Master is currently ranked 18th from the bottom.
And players joke that Azir finally has a decent rival.
It can be expected that Riot will rebuff Master in a few days or in the next update, and will continue to do so until the hero reaches an acceptable level.