This doesn't apply to third-party stores I think. If this works anything like other prompts of this kind, this will only appear for apps installed directly fro

This doesn't apply to third-party stores I think. If this works anything like other prompts of this kind, this will only appear for apps installed directly from an apk file, but not via apps installed from a store, even if the store itself was installed through an app file. In principle, it would be possible to create a store that installs apps from the file system, perhaps with some affordances to enable automatic updates from a URL. I don't think it could support app files directly, there are some extra requirements on store "bundles", but an "installer" should be possible.