Apple restricts new Swift concurrency features to iOS 15 and above only

Most places I've worked support at least a couple OS versions back. Lately because of when the last device support cutoff was, it seems to be iOS 12. That's a long time to have to wait to use any of this, especially given the enormous benefits and how much more efficient it could make developers. Yes, we could #if available, but that would end up doubling up the work you have to do, completely nullifying most of the benefits.

Sounds like they are considering adding back support, but personally I'm a bit surprised that isn't set in stone since these are some game changing features.

Does sound like there would be a lot of technical limitations to adding support in prior versions.

Interesting discussion of the behind the scenes here https://developer.apple.com/videos/play/wwdc2021/10254/