The CSP header plugin  has not seen any activity by its author since
I submitted a PR to update it back in 2018 , which got no response
despite reminders. Then Andi sent another PR in September 2020, but he
didn't have better luck.
I have tried writing to the author, Matthias Schulte (aka lupo49), a few
times, but received zero response (and no error message either). So this is
one last try to reach out to him - maybe he reads this mailing list, or
someone knows how to contact him...
Without response, I am considering to fork the plugin and provide
(minimalistic) maintenance for it, unless - per Michael Hamann's suggestion
 - it can be moved to DokuFreaks org.