Yuumi's rework to try and dissuade her from being OP in pro play will be going live in LoL Patch 13.5. Unfortunately, it will be going live without some of the intended sound effects due to some nasty in-game bugs.
These bugs only affect certain skins on the champion though, so don't worry if you're going to test out her new update in this patch without a skin you should be fine.
Yuumi Rework: Some SFX Changes Pushed Back to LoL Patch 13.6
Yuumi has been a menace ever since she was released. In recent years she has dominated the pro meta and Riot is making big changes to try and shift her to a low elo only champion and a champion to try and get people into League of Legends.
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They changed her entire kit around, keeping her untargetability as-is though. But most of the other abilities received major overhauls, as well as her passive. If you're curious about the changes make sure to read our LoL Patch 13.5 patch notes article.
Two skins will have incomplete SFX upon release though, with her EDG World Championship R, as well as her E on Yuubee being way too loud. Players were unable to hear anything but the sound of the SFX, which is why Riot chose not to ship Yuumi with those awful sound effects.
For now, Riot has added some temporary SFX and shipped her with that, but they will adjust the volume and then give Yuumi the intended sound effects in the next League of Legends patch.