yes that would be the easiest but it depends on the notification type...If users and whoever is destined to receive them can no longer interact with the course, there should be no issue to leaving it attached to the notification.

Agree that we need a much better way to manage notifications as you can only remove them manually. It helps if you know which courses as you can just teach for them but it can be a lot of manual work. If you add some text or codes when you deactivate a course, you can use that text to find them more easily in the lists (a date works well too); or we use “RTD” when a course is retired...