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Google's annoying 'update ownership' sideload warning spreads to more apps and phones
(www.androidpolice.com)
This is a most excellent place for technology news and articles.
My understanding is that this dialog won't appear if you update apps from f-droid or other 3rd party store, but only if:
That would normally fail because the signatures don't match, but I guess there are places, like apkmirror, where you can find apks that are signed by the original developer (and so, they are authentic and 100% safe), but then google will give you a warning that you might not receive updates and such.
At least, that's my understanding. A tiny bit annoying, but not a big deal.