Because your previous request didn’t get traction with a developer who had time or motivation to create it?
Or F-Droid and Android Installer already do it for you, without giving you the “proof” you wish to see. Have you tried installing an app apk from one source or F-droid repo’, then attempting an update from another? You would find it will complain about signature mismatch.
Or ClassyShark3xodus already shows you the info.