You might sideload an Android app, or manually install its APK package, if you're using a custom version of Android that doesn't include Google's Play Store. Alternately, the app might be experimental, under development, or perhaps no longer maintained and offered by its developer. Until now, the existence of sideload-ready APKs on the web was something that seemed to be tolerated, if warned against, by Google.
This quiet standstill is being shaken up by a new feature in Google's Play Integrity API. As reported by Android Authority, developer tools to push "remediation" dialogs during sideloading debuted at Google's I/O conference in May, have begun showing up on users' phones. Sideloaders of apps from the British shop Tesco, fandom app BeyBlade X, and ChatGPT have reported "Get this app from Play" prompts, which cannot be worked around. An Android gaming handheld user encountered a similarly worded prompt from Diablo Immortal on their device three months ago.
Google's Play Integrity API is how apps have previously blocked access when loaded onto phones that are in some way modified from a stock OS with all Google Play integrations intact. Recently, a popular two-factor authentication app blocked access on rooted phones, including the security-minded GrapheneOS. Apps can call the Play Integrity API and get back an "integrity verdict," relaying if the phone has a "trustworthy" software environment, has Google Play Protect enabled, and passes other software checks.
Graphene has questioned the veracity of Google's Integrity API and SafetyNet Attestation systems, recommending instead standard Android hardware attestation. Rahman notes that apps do not have to take an all-or-nothing approach to integrity checking. Rather than block installation entirely, apps could call on the API only during sensitive actions, issuing a warning there. But not having a Play Store connection can also deprive developers of metrics, allow for installation on incompatible devices (and resulting bad reviews), and, of course, open the door to paid app piracy.
I would be really happy if you're right, but I sadly think Google's fine here. As far as I understand it, this particular regulation is to prevent a powerful actor (Google, Apple) to use their monopolistic powers to shut alternative stores down. It's not about allowing customers to install whatever and however. Google doesn't shut anyone down with this, so they should be fine. They give the option for app developers to choose if they want to run only on an attested platform - which they sell as a completely optional security feature that nobody has to use.
My guess is if the EU is going to take this further it would have to be regarding a potential monopoly on the attested platforms on the device. Google only offering their own platform as trusted could potentially be seen as another monopolistic behavior. If we're lucky.
The problem is though that the attested platform only accepts Google play as a store, for this to be truly fair you'd need a way to set a default store setting up and then the attestation API checks that store, but as things currently are it's giving Google play store an unfair advantage.