The thing is that there really isn't anything SwiftUI brings to the table from an application standpoint that you can't do with UIKit. (Albeit with a lot more code.)

Our Android team has a cutoff of Android 5, but that's really only because they hit a limitation that required it.

Offically, and from a testing statepoint, we're at current version minus 1 on iOS and minus 2 on Android. But we don't drop older clients unless and until needed.