I submitted two test reports, both for Xubuntu 18.04.4 which is a current LTS for Xubuntu. I got an email:
Subject: [AppDB] Submitted operating system deleted
Submitted operating system deleted
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The operating system you submitted (Xubuntu 18.04.4) has been deleted.
Apparently you tried to submit Xubuntu as a new distro version. It isn't; Ubuntu derivatives are all grouped together under Ubuntu, and that's what you should select when submitting test reports for Xubuntu. As for the "Do not use," that's just an admittedly cludgy way to deal with incorrect versions, as the current system does not allow them to simply be deleted (they have to be merged).