Android apps have a flag in their manifest which tells the OS "this app was built with Android X (API level X) in mind". This allows the OS to selectively enab

Android apps have a flag in their manifest which tells the OS "this app was built with Android X (API level X) in mind". This allows the OS to selectively enable backwards compatibility and change certain behaviors (e.g. selectively enforce new permissions so old apps aren't broken). Play Store requires apps to target new OSes and port APIs within certain time of an OS launching (usually ~2 years). This avoids apps targeting older OSes to avoid new security and privacy enhancements (e.g. asking for permisisons to show notifications, asking for permisisons to access microphone, being allowed to show a fullscreen popup ad, etc. Those restrictions were all gated behind the target check.)