A user added the "event" content type to a list on SharePoint Online. It installs some columns that are any combination of un-deletable, hidden, having dependencies each other. After deleting the "start date" column, the list becomes in-operable. Items cannot be added or modified. Removing the event type from the list does not allow you to clear these columns out, adding the columns back does fix the issue where items can be added/modified.
I'm in the trenches with PnP Powershell, but I may have to delete the whole list and start fresh.
I don't even know where to officially report this issue, but I'm posting it here so maybe someone else can avoid the nightmare.