I have had schedule changes on LX, LH and BA in the last 12-18 months and was notified NONE of the times...
I regularly check my itineraries and that’s how I spotted these changes/cancellations, but I really don’t know how long it would have taken them to tell me...
some of these were not minor changes, they were canceled flights, even one led to my ticket ‘disappearing’ (not showing up anymore on the airline website)
so, based on personal experience it’s many airlines who have this problem and there seems to be nothing that can be done about it except being super vigilant.