Threema has just released Threema Libre.
According to their FAQ: "Threema Libre is a Threema version for Android. It’s completely free of proprietary dependencies (i.e., it’s 100% open source).
According to Threema Libre: Full Independence from Google Services
" In Threema Libre, there’s no single line of code that would require a proprietary software library from Google or any other third party. Push notifications, for example, will only be delivered using Threema Push, and a fallback to Google’s push service is ruled out from the outset. Since Threema Libre also supports reproducible builds, it’s comparatively easy to verify that the installed app cannot leak any data to Google. And because the whole code including all components is open source, the app can be audited in its entirety."
Source code is available here:
The Threema Libre flavor is build variant libre, see:
Not yet free of Google, maybe soon: Threema is open source now (#1596) · Issues · F-Droid / Requests For Packaging · GitLab