From some tests, my colleagues have made, it’s not that sonar.branch.target does not work at all. It seems to be considered for “New Code” recognition for short-lived branches and for issue synchronization (for both branch types).
The workaround we’ve decided to use for long-lived branches, is to use the “Previous version” setting for “New code” in the project, then make sure, that the first scan for a certain branch is actually made for the base branch state of it and only the subsequent scans to be of the actual branch state and then manipulate the version numbers in such way, that the initial scan (so of the base branch state) is considered the previous version for this branch.
This is a crude hack, very tedious and error prone. Also for some of our repositories, this seems to cause a different issue to surface: the issue of new code not being detected (forcing us to use sonar.scm.disabled=true):