I didn't think it was available on the official F-droid repo yet? It is on IzzySoft's repo though - Joplin - IzzyOnDroid F-Droid Repository
I guess there are still unresolved proprietary dependencies.

As for the versions, the release notes will likely state if there is going to be a problem due to a change. For example when Joplin hit 2.3 it upgraded the sync target itself which required all clients to be on 2.3 which was 1) in the release notes and 2) in the error messages if a device attempted to sync.