calculus
I'd say that about the first half of Calculus I was useful to me, taught concepts.
However, a lot of the rest of it, as well as most of the next two calculus classes I took, involved memorizing tricks to do symbolic integration by hand. That is, frankly, of limited use to even people who need to do symbolic integration.
I remember going by my calculus professor's husband's (another math professor) office once to deal with some project I was doing and some integration came up and he promptly threw it into Mathematica on his computer to do it. I commented on it and he said "yeah, I don't have time to spend doing these by hand".
My smartphone and computers have Maxima installed, a free and open-source computer algebra system capable of doing symbolic integration. I have that with me all the time. It's very rare that I need to do symbolic integration in the first place.
"But what if you don't have a calculator with you?"
Today, that's usually a smartphone, but same idea.
In my parent's generation, they taught people to manually compute square roots. It was some numerical approximation, don't know what they did exactly, probably something like "pick a number, divide, average result and divisor, repeat with average". They didn't bother to teach that by the time I was going to school. It was just expected that you'd use a calculator.
I remember reading Richard Feynman's book, about how he used to show off some mental math shortcuts (much more useful in an era before calculators).
I agree that memorizing certain mental math processes can be useful, but the time wasted on doing symbolic integration in calculus is still one of my major annoyances, looking back. There is no shortage of material in mathematics that is useful and could be in the curriculum, and instead we did symbolic integration.
Maybe curriculum has improved since then.