I suspect the vast majority of non notifications on flight changes are simply timing. As airlines make schedule adjustments they wait until they feel the new schedules are locked in before rebooking and sending notifications. Consumers who happen to see their flight canceled before the process is complete feel they were not going to be notified which is not the case. The fluid situation with the max is making the situation worse.
As a new delay on the max is announced those flights are pulled from the schedule. The airline then starts looking at options such as extending airframes planned for retirement, extending leases, sourcing additional airframes, deferring heavy checks ect.. It can take several weeks to get a final plan. At that point rebooking would be completed and notifications sent shortly after.
One last thing, check your junk mail. It can be surprising what gets pushed over to that inbox. A friend was enraged JetBlue did not notify him of a change. It was in his junk mail.
G