They do maintain compatibility, it's just obviously running in compatibility mode because of hardware changes.
If your app doesn't account for a notched display, or a retina screen, or multi-tasking or whatever, then there's nothing Apple can do aside from run it as it used to run on older hardware, without supporting the new features.
If your app doesn't account for a notched display, or a retina screen, or multi-tasking or whatever, then there's nothing Apple can do aside from run it as it used to run on older hardware, without supporting the new features.