For reference, things known to humanity in 1940 (though not necessarily understood, that was an important difference in my comment) include the Schwarzschild (stationary black hole) and FLRW (homogeneous expanding or contracting universe) solutions to the equations of general relativity (1917 and 1924 respectively), the Darwin term for the effect of special relativity on the spectrum of hydrogen (1928), the severely non-classical exchange interaction responsible for the existence of ferromagnetism (same year), the London equations describing the (non-)penetration of the magnetic field into type-I superconductors (1935), and a way to produce superfluid helium (1937). And that only includes things I was supposed to ( :( ) know at some point. Also Gödel’s incompleteness theorems (1931) and Turing’s proof of undecidability of the halting problem (1936). I’m certainly not proposing to demand high schoolers know all of those.

The only reason I had to go as far as 1940 or so is because linear algebra came relatively late in the history of mathematics, even though it should logically serve as the foundation of multivariable calculus—and was enthusiastically adopted as such once it finally entered the collective mathematical unconscious in the late 19th century. Working out the pedagogy took a couple more decades.

If we were only talking about the “advanced” mathematics of calculus, I’d say Newton’s knowledge as of 1690 would be way overkill. (That would include the convergence speed of power series, a decent theory of ordinary differential equations, the beginnings of the calculus of variations, and even the “Newton polygons”, a theory of formal-series solutions to polynomial equations that properly belongs to algebraic geometry.) At that point J S Bach was five, the city of Philadelphia was a town founded eight years ago, and Peter Romanov (later called the Great) was still twelve years away from founding St Petersburg and thirty years away from proclaiming Russia an empire.

To be honest, I think even if school taught no mathematics beyond basic numeracy, I wouldn’t complain as much. I would still complain, mind you, but only to the extent I do about being able to grow up without knowing what a fugue or the twelve-bar blues is or who Giotto or Niccolò de' Niccoli were. Instead we torture people with years of alleged mathematics bearing no resemblance to the real thing, and thus give them license to think they are not “maths people” (whatever that means) and that whatever maths they did not hear about in school is “advanced” and obscure. At least the average high-school graduate knows he doesn’t have a clue about music history or theory.

My tirade had a point as well, though: post 20th century our collective intuitions have become wildly miscalibrated regarding which things in humanity’s understanding of the world are recent or obscure. It seems that the barrier of “advanced” mostly hasn’t shifted at all since high school became compulsory in most parts of the world, and as decades go by I can see this going from “a goddamn shame” to “existential threat to human culture”. I’m not even sure that hasn’t happened yet—already a high school teacher can rarely, if ever, thoroughly explain a recent development of their choice in their subject.