If you're interested in running free and open-source Android applications, then you've probably heard of F-Droid. While its limited catalog of apps makes it far from an actual alternative to the Google Play Store, F-Droid is the only app source out there that contains nothing but open-source Android apps. Every single app published on F-Droid's official repository has to be completely open-source, meaning they can't include any closed-source components. With Google's recent requirement for developers to shift to Play Billing Library v3, developers with open-source projects on F-Droid are facing a problem. Per XDA Recognized Developer M66B, the developer of NetGuard and FairEmail, developers who publish apps on both Google Play and F-Droid will need to start building a separate version of their app without the Play Billing Library.
Google Play Billing Library version lifecycle. Source: Google.
So why is this happening? As it turns out, Google stopped uploading the source code for its Play Billing Library after version 2.0.3. There have been 4 versions since 2.0.3 that are, thus, closed-source. This has not been a problem until now since apps could use the older Play Billing Library v2 just fine, but since Google will soon require developers who publish apps on Google Play to move to v3 (still closed-source), that's where we start to run into problems.
There is no free software method for Play Store purchases: Developers need to use Google's libraries in order to allow users to make purchases through Google Play. Developers shouldn't have too much of an issue building a version of their app with the Play Billing Library for Google Play and a version of their app without the Play Billing Library for F-Droid since Gradle allows product flavors with different source code sets, though. However, this change will be a bit inconvenient for users downloading apps from F-Droid as they won't be able to use Google Play for purchases. If you don't have the Google Play Store, then this may not be much of a problem for you since you likely don't use Google Play billing anyway. If you use F-Droid as merely an alternative app provider, then the apps that will be affected by this change are probably also available on the Google Play Store.
https://f-droid.org/en/packages/eu.faircode.email/