Threema is a Swiss messaging app, that was particularly well-known in German speaking countries. It was well known for it privacy focus. Only downside - until now - was the fact that the app was proprietary. As of today, it seems Threema is open source software. Here is the company announcement:
At least until now Threema is not for free, it costs a one time payment. Therefore the app is NOT freely available in the /e/ AppsStore - but it can be purchased directly on the company’s website (or on Google Play).
In terms of its privacy focus, sole major competitor (I believe) is Signal. For further reading, check out this website: https://www.boxcryptor.com/en/blog/post/encryption-comparison-secure-messaging-apps/