Glad to see this thread here, I was going to start my own but then found this one.
I tried installing the Finnish Koronavilkku app on my XA2 running Sailfish OS 3.3 Rokua. I didn’t find it in Aurora Store for some reason, but was able to download the APK from APKmirror. In addition, I have installed the MicroG package (reimplementation of Google Services) which contains an implementation of the Exposure Notifications API in the newest release. The Koronavilkku app started up, but it doesn’t work - it just shows a big red “Ota käyttöön” (‘start using’) button which doesn’t do anything. This was what I suspected, but I wanted to give it a shot. I believe the missing piece here is the Android emulation layer which has extremely limited support for Bluetooth (speakers only IIRC). So even though the rest of the stack is there with MicroG implementing EN, it can’t talk to the BLE hardware. Assuming the Bluetooth support for the Android emulation is not going to improve (and I don’t think the community can do much as it’s proprietary), this is a dead end.
That leaves the option of a native implementation, which was already discussed above. To add a few pointers to the discussion, I found this blog post by Alyssa Rosenzweig interesting: it describes the author’s Linux implementation of the Exposure Notifications service/API (called liben) as well as a Linux application that attempts to be compatible with the Canadian COVID-19 tracking app / infrastructure. Technically it’s a success although there are problems with the keys needed to contact the official key servers.
If there are issues with other implementations of Exposure Notifications, maybe liben could also be of use on the Sailfish platform?