> Is it hidden if it's explained in the standard?

In the context of that particular complaint, yes. From what I understand the gist of it is basically that you should be able to tell what is going on by looking at the code locally (i.e., the code is "explicit").

> I think Linus's complain was before there was a c++ standard.

These emails [0]? IIRC those are the most well-known ones and they are from the mid-2000s

[0]: https://harmful.cat-v.org/software/c++/linus

An empty loop, under some non-obvious conditions, on some compiler flags but not others, silently transforms into a system call. In a systems programming language.