They're rejecting your reports because the issues are often fixed in newer versions - someone would have to re-confirm your reports on a newer version.
Sure, newer versions fix bugs from older versions. But they don't delete/archive the old reports after a new release, so the reports that are there are already a mix of supported and unsupported versions anyway.
So, I don't see the harm in accepting new reports against older versions, particularly when old versions are still found several major distros (particularly the LTS versions of those distros)
I run it on top of Debian testing.
Maybe the policy about rejecting debian reports came way before they setup the repository.