You may be aware of Google's Advanced Protection Program, which allows customers to opt-in on additional security protections and restrictions for their Google account, like a forced two-factor hardware security key requirement without fallbacks, and reduced account access for third-party apps. In addition to enhanced security, it also comes with other recent perks. According to details revealed in a recent teardown by 9to5Google, the Advanced Protection Program may expand to blocking apps from being installed from outside the Play Store.
Before you get up in arms about this, we should reiterate that this change, as currently described, would be part of an optional, opt-in program that users can elect to use to enhance their account security. This won't affect the vast majority of our readers, and Google isn't going out of its way to further lock down Android as a platform — at least, not with this. The entire point of the Advanced Protection Program is the extra security measures it imposes to enhance security, and this would be a very logical and consistent addition. If you don't like it, just don't use it.
According to the details of the teardown, the latest version of the Play Store (version 17.8.14), includes a handful of references that state, pretty explicitly, "For additional security, Advanced Protection won’t allow apps from outside the Google Play Store," though another detail dug up implies that Advanced Protection may also scan apps via Play Protect prior to sideloaded installation. Based on that apparent inconsistency in the app's code, perhaps even this blocking of sideloading apps might be an optional setting, or this exemption could be in place for businesses and other organizations using the Advanced Protection Program that still require sideloading apps. Also note that G Suite already allows for administrators to block sideloading/installing from "Unknown Sources."
Since this info comes courtesy of a teardown, details are still sparse and subject to change. We'll likely see more about it in future teardowns. And again, before you get too worried about downloading apps from APK Mirror, by all current indications, this will be purely an opt-in change.
Google has since instituted and begun enforcing this change.