It seems useful to educate C programmers who don't have "significant experience" because, well, by definition, not every C programmer has "significant experience" and they are the ones who would benefit most from learning this.
It's not about being controversial: nobody's going to disagree with your explanation (it is what it is - it's not an opinion).
So I would say... What good is a verbose comment to explain? Somebody who doesn't instantly understand why it's banned can read the big warnings on the manpage, or they can google it and land on good explanations on sites like Stack Overflow, or they can look at the function signatures and come up with a correct guess. Then boom. They know. And from there they can assess other interfaces and see if they suffer the same weakness.
This ability tends to come from simple exposure.
Linkers already warn for this stuff. Doing it this way requires every source file to include banned.h, which I would guess is done by including it from some other common header, but that's not fool proof either.
Competence can be measured objectively. Failure to understand array bounds is prima facie evidence of lack.
Seriously, just looking at function arguments shows you, there is a problem under the hood. Reading docs also will point out.
It is open-source, read and learn from Git, Linux code and so on. If you do not have experience is sysdev or high security, just don't contribute there.
This is like saying, construction engineering is gatekeeping beginners, because you have to work for few years before you can start your own project.
Edit: I guess some down voter doesn't believe me but it continues to be true. They are all string functions. Most of them do not take an output buffer size, so a source string exceeding the destination buffer will overflow. Others, like strncpy, take an output buffer size but will not null terminate when exceeded, so the buffer size must be reduced by 1 by the caller and potentially manually terminates, which is too easy to not consider. But any C programmer with significant experience already knows these pitfalls. So it's not controversial.