Is new SwiftUI API backward compatible with iOS 13?
Apple documentation includes this information with version support noted. Look at this example: https://developer.apple.com/documentation/swiftui/lazyhgrid The information is on the right-hand side.
Neptunes gives good advice about finding this information, but to answer this more concretely: No, the documentation indicates that the new APIs are not backwards compatible.
That's a little disappointing. Each year, with newer controls and improvements, one would need to bump the supported OS version, which isn't always feasible. I'm hoping moving forward people would probably just stay on the latest and greatest version of macOS but it may become an issue with iOS. Adoption can still be slow at times.