I used to work at a travel company that booked packages that included airfare. One of my friends worked in the Air Department. That was the department that handled schedule changes.

According to her, the airlines typically meter their notifications about schedule changes to avoid overwhelming the call centers. The notifications are sent out in reverse chronological order. So, a schedule change for next week will be sent immediately but one for five months from now will be held in the queue.

All notifications for a given flight are sent at the same time. In theory, that means that everyone affected will have roughly the same options for re-routing.

Additionally, airlines will hold off on sending notifications until they are reasonably certain that the schedule is finalized. For flights many months in the future, there might be several changes made before the right aircraft is placed on given flight. Rather than sending out notifications after each change, they will wait until everything is finalized. Doing that limits the annoyance to customers.

Given the state of the world, no airline is certain what their operations will look like next fall. As such, I'd guess that they are delaying virtually all notifications of schedule changes.

As for why people get notifications for changes of 5 minutes but not 5 hours to a flight, I'd speculate that is due to the fact that a minor change isn't going to result in a flood of calls. As such, they send out those notifications right away.