Theema is based in switzerland. And the swiss authorities and the swiss army communicate via Threema. They would be stupid to shut down their own communication-ways!
https://breakingthe3ma.app/files/Threema-PST22.pdf What changed since this paper about Threema holes in his security model?
Everything metioned there is obsolet. Threema implemented a new protocol! Also see: https://blog.dbrgn.ch/2023/1/14/threema/