>Basically, classes that provide operator[] provide at

std::span does not provide at.

It doesn't seem to be an oversight because P1024 proposed it but it was
voted down (0 strongly for, 0 weakly for, 4 neutral, 4 weakly against, 5
strongly against).

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1024r3.pdf

So to me it seems like at() has perhaps fallen out of fashion, at least for
sequential containers.