Those aren't the OS, those are individual apps that have chosen to use a web rendering engine for various content, and it's not even correct. Calendar doesn't use WebKit at all and never has. Mail.app uses it to render external HTML input (i.e. email messages) rather than UI. I believe Music.app moved off WebKit some time ago as well, and it looks like App Store did too (Accessibility Inspector confirms that the Updates page, which used to be rendered with WebKit, is now a collection view). Both Music.app and App Store link against WebKit but that's likely because the Account Settings page is rendered with WebKit (this is likely content that is served remotely).

In any case, the OS itself does not rely on WebKit for UI, it just ships some apps that use WebKit.