Drupal latest forum on this topic: Drupal 9 readiness [#3130013] | Drupal.org
What should I do?
Actually, after a bit of investigation, we found out that this package was being used by some views that were disabled.
We found that out by trying to remove the package.
Before you the package is actually removed, it will say where it is being used.
We checked the views, we saw that they were disabled for some reason, so we just deleted the views and then delted the package, so there was no need to collaborate with anyone.
It seems that the people who were developing this site were playing around with different packages and some views were simply forgotten to be cleaned up.
I initially wrote directly to the maintaners (on the drupal forums) but nobody was in a hurry ¯_(ツ)_/¯