Either that or it should indicate to the end user that it's an unsupported package. Maybe showing up as KeePassXC-Debian or KeePassXC-Unsupported and have the developer's contact details (website etc) removed from About and replaced with the Debian details for support.
A downstream maintainer making small changes to fit within the OS that doesn't meaningfully affect the app is fine. A downstream maintainer modifying an app and removing core functionality so the upstream dev gets a ton of grief is not.
A downstream maintainer making small changes to fit within the OS that doesn't meaningfully affect the app is fine. A downstream maintainer modifying an app and removing core functionality so the upstream dev gets a ton of grief is not.