In the post announcing libadwaita, it said that the Adwaita stylesheet "including its HighContrast and HighContrastInverse variants" was going to become part of

In the post announcing libadwaita, it said that the Adwaita stylesheet "including its HighContrast and HighContrastInverse variants" was going to become part of the library. So don't worry, it's not going anywhere! GNOME as a project does put a lot of effort into accessibility, they're not just going to throw those themes away. I think the key realisation here is that they consider them to be Adwaita variants, not entirely new themes.