As the title said. I had done some search but most of them seem for users not for developer. Any way for checking compatibility or finding breaking changes between iOS versions?
My app is targeting to iOS 10 and now I want to change to 13 for new app upload due to "Starting April 2020, all new apps and app updates will need to be built with the iOS 13 SDK and support the all-screen design of iPhone XS Max or later." https://developer.apple.com/news/?id=09102019a
How to check whether my swift code is working fine in different iOS version？
You have many ways:
To explain further, Apple's note, which you seem got a little bit confused of:
Starting April 2020, all new apps and app updates will need to be built with the iOS 13 SDK and support the all-screen design of iPhone XS Max or later.
It only means that you should build your app with iOS 13 SDK - which does not necessarily mean you need to make your app/project target a minimum iOS 13.0 version.
See my answer here: https://stackoverflow.com/a/62918044/3231194
