GMS2 versions 2.2.4 and above target the newer AndroidX system of common functionality, rather than the older Android Support Library system. Accordingly, any extensions which use the old Support Library now need to be updated in order to remain compliant with Google Play requirements and the expectations of the GMS2 2.2.4+ runtimes.
Note: If you're using 2.2.3 or older runtimes, then it's still the old Support Library which is to be used and you can stop reading this FAQ after this note. This will stop being supported by Google Play in the not-too-distant future, however, so you will have to update your extensions soon...
Please be aware we don't offer custom advice for how to port a third-party extension to the new Android build system, and so we won't handle support tickets about this (you will just be asked to make a GMC thread with your questions), but here is a list of resources you might find useful if you have issues updating your extensions. This page will grow as we hear of issues and find more resources we can highlight.
Always remember, if you didn't write the extension it might be quicker and easier to ask the original author to update the extension. Then, everyone who uses the extension can get the update from them.
We will update our own official extensions on the Marketplace when 2.2.4 is fully released. They will be available sooner as direct downloads on the 2.2.4 beta threads on the GMC.
Can I use AndroidX with SDK Manager? No. Google don't offer this as a download in SDK Manager.
AndroidX overview and docs homepage: https://developer.android.com/jetpack/androidx
Migrating to AndroidX: https://developer.android.com/jetpack/androidx/migrate
Support Library to AndroidX artefact mappings table: https://developer.android.com/jetpack/androidx/migrate/artifact-mappings