My experience is that GT teachers have much higher expectations. There's no "easy street" in AP classes, and GT classes are the functional equivalent for earlier grades.
I agree that public schools used to be more challenging in some classes, but back then public school wasn't mandatory.
Of my four grandparents, only my dad's side graduated high school. My mother's mother only finished 8th grade, her husband only 4th. Strange because the brains are on my mother's side, but they had to work and it was considered normal not to continue.
Also, I doubt that most public schools back then were teaching calculus, much less matrix algebra or differential equations. Not to mention microeconomics, macroeconomics, anthropology, psychology, astronomy, etc. being taught by Ph.D.'s in the field.
Which is a small fraction of the classes offered at my kids' high school. Every time I read people complaining about public schools, I can't help but wonder what home schooler or private school can offer all this and more. Some very elite private schools do, but not many.