> Presumably this one can be triggered on an already installed app - a key question being how that triggering occurs. i.e. will the Play Store act to push out details of developers that are now blocked so devices can act on it?

Your "presumably" is doing a _lot_ of work; these strings are from the PackageInstaller, and go along with all of the other reasons you can't install an APK.

Historically, apps that were pulled from the Play Store and developer accounts revoked due to malware do _not_ affect apps on the end-user device, and there's no current sign of this changing with this specific project. Google have generally achieved this goal using Play Protect, the separate app/service which _can_ download revocation lists and signal end-users to delete malicious apps, and there's no indication this will change.