I am recreating annually previously archived and deleted courses, and in about 10% of the time (10 out of 100) I receive this warning. So far I have not seen anything else wrong, but I do wander if archiving/deleting some courses did not work completely.
I just deleted a course and noticed that in the admin course class list editor the professor account of the deleted course is still there, but instead of Enrolled status it now shows Drop status.
Manually deleting the professor account from the admin course eliminates the warning.
I don't see what the reason is to keep a deleted course's professor in the database.
We have talked about this, and I think that we are going to stop creating a user in the admin course entirely. There really is no point in these users being there. They have no real purpose.