This is the stock SMS app from AOSP, configured as an Android Studio project so that it can be built without having to download and compile all of AOSP. It builds with Gradle Plugin version 3.5.3 and Build Tools version 28.0 using Android Studio 3.5.3
The goal is not to fix and/or enhance this Google et al. owned app outside its premises, but rather study, observe, experiment with and other reasons you can think of. Source code changes, if any, are kept at a minimum.
There's a long list of branches and tags upstream and I really don't know what they refer to, which ones are stable (if any). So far I've been experimenting with several tags/branches, none of which proved to be stable. "App Vitals" page of this app in my Play developer account is flooded with reports of ANRs and crashes, no matter which tag/branch I follow. Caveat emptor!
Having said that, currently there are 4 branches in this repo:
9.0 follows a recent android-9.0.0_rXX tag from upstream (check ext.verName in top level build.gradle).
Likewise, 10.0 follows a recent android-10.0.0_rXX tag from upstream.
sdk-release is a sync of the same branch upstream.
goplay is a fork of sdk-release with modifications required by Google Play (yes, Google finds its original app unacceptable to Play market).
I reckon 9.0 branch is the closest to original app. It should be as stable as Google et. al intended it to be. There is an APK of this branch under releases section (but it is not possible to submit this revision to Play market).
sdk-release has the most recent commits and is currently ahead of both 9.0 and 10.0.
In order to be accepted to Play market, goplay branch bumps targetSdkVersion to 28 which causes notifications to disappear on Android 8+ and intermittent crashes, because of the backward-incompatible changes "Android Team" has progressively been introducing to the SDK.
Currently android-10.0.0_rXX tagged revisions at upstream exhibit a "seemingly premature" targetSdkVersion=28 setting in their AndroidManifest.xml, as modifications required to handle notifications for that level were not observed in their sources. This is reflected at 10.0 here.