Message app (qksms) - no backup possible

Still impossible a this date to bakcup with the /e/OS native version. Still needed to install the F-Droid version to do it.
I do not understand why it’s not possible with the native version (fork of QKSMS) when QKSMS is an Open source app ?

It is possible, but the /e/ developers will need to pull in the latest version from QKSMS, and I guess that is not high in their list of priorities. They don’t seem to have merged in code from upstream since July 2020 (or maybe earlier) since which time upstream has had lots of commits
https://github.com/moezbhatti/qksms/commits/master

This is the downside of forking code instead of using upstream directly: the fork will always lag behind upstream in terms of new features and bug fixes.