Perhaps, not the solution you are looking for, but you are able to run a virtualized android session on your phone, and through that you can install Threema. This isn’t all that realistic, even though it completely works, because it is a resource hog and battery drainer. On top of that, you would need to restart the android system everytime you wanted to use it. Push notifications are not transparent to phosh, and so it would work, but you would need to physically open the app through android everytime you want to send or check for messages.
Thank you for your answer. Could you point me to instructions for doing this? While it surely does not sound like an ideal solution (whatever happened to the chatty plugin?) I’d like to give it a spin. After all, that’s what a librem5 is for - tinkering 
If you own an iOS device as well, you could try the Threema Desktop 2.0 tech preview:
It runs as a standalone client on Linux. Keep in mind that it’s still beta though, and that the application is optimized for Desktop use, not for mobiles.
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I tested threema through waydroid.
It works, but without camera access ( Anyone Successfully Run WayDroid on Librem 5? )
I followed this way:
- Install waydroid ( https://source.puri.sm/-/snippets/1198 )
- Then in waydroid : F-droid ( https://f-droid.org/ )
- Then in F-droid install Threema-libre ( https://threema.ch/en/faq/libre_installation )
Share us your feelings !
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Thank you very much for your instructions. I tried this and it works! Having Android running inside PureOS is quite the feat 
However, I’m having power issues and thus refrain from using waydroid as an always-on app. Maybe someday later, when the battery optimizations of L5 have further improved.
Still, having Threema and proper XMPP in chatty would be fantastic.
(I remember having the same half-usable XMPP telepathy solution with my Nokia N900, and it seemingly hasn’t improved since then.)
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