During a written Q&A (opens in new tab), one player asked why the developer had given Kai'Sa an "unnecessarily deep neckline", given that it had previously promised to avoid releasing hyper-sexualized champions. In response, Riot's lead producer of champions Reav3 said: "We’ll be the first to admit that Kai’Sa’s neckline didn’t land well with players and that it made her look unnecessarily sexualized, even if that wasn’t the intention."
Early on in designs, Kai'Sa didn't have a deep neckline, Reav3 said, but the team decided she looked too much like a Void creature or a human that had been corrupted by the Void, rather than just the "normal person wearing a Void-suit" that they were after. The team's solution was to show more of her skin.
"The human skin showing on her face was small on screen and difficult to read from the in-game camera. Our artists tried a few things to try and address this, and the one that worked well was redesigning her neckline so you could see her human skin more clearly when her mask was off."
Reav3 goes onto admit that Riot should've "prioritized searching for other ways to solve this problem", before adding: "we could’ve done better, and in the future, we will be even more conscious of these decisions."
However, Reav3 made no mention of retrospectively changing Kai'Sa's design, so it looks like she's going to look the same going forward.