I thought developers could bundle Swift with their app? So even if iOS has version X, you can bundle X+1 and it’ll work.
For example, when you run an app made in the iOS 15 era on an iOS 13 device, the app can’t use any runtime features that are not included with iOS 13. Given that swift is still rapidly evolving, that amounts to a lot of useful features. This year, iOS 16 apps get to use the cool new Regex features.