Checking other posts iOS 12 should still be supported but the having upgraded an App in App Store it will not run on iOS 12 devices. Is there a trick to building for iOS12 support or has the goal post moved to iOS 15 which I find in specs for XCode
XCode 16 and iOS 12 support
iOS 12 is not supported by Xcode 16. The Xcode Support page has the details.
— Ed Ford, DTS Engineer