I think the rot started to set in from the early 1990's.
That was around the time the briefing offices @ the then GAAP aerodromes started to close. Many pilots including students and instructors used to wander in to the briefing office and ask questions of ATC & FS staff, who in those briefing roles and locations really knew their stuff. Along with the MET man, of course.
That availability of face to face knowledge base disappeared, people were left to their own interpretations and what they'd been taught, and here we are.