Earlier quoted context omitted.
I don't get why calculus is always brought as an example. It wasn't particularly hard, the entire class had to learn it in high school. We all had it (in a slightly harder shape) in every university course (no matter how detached from what we actually needed) I forgot all my calculus after high school, had to relearn it in uni and then I promptly forgot again. Exactly as the article says, it was more about proving we…
I think that it is a US specific obsession. I don't even know what "calculus" is really. I have had plenty of math classes both in high school and later at university but I don't recall any significant distinction that would leave me with some concrete idea of "algebra" vs. "calculus" vs. "whatever" years later.
It's the continuity/limits/integrals stuff.
Also it was never optional. Either in high school or CS at uni.