Following a lengthy discussion on the topic, the first step
towards getting themes cleaned up and hidden when they don't
work is to get the ones hidden which are only for Plasma 4.
We do this by filtering out any theme which is tagged as
being for the major version 4 of plasma. We also allow themes
through which have been tested to work on both, by accepting
themes which are tagged as both 4 and 5.
 https://phabricator.kde.org/T8126#184198 and down a few pages