That's not true. People don't staunchly avoid situations that have pointless sharp edges. Putting up caution tape, despite everyone being aware of the danger, is very often a useful activity.

I actually wonder why they do it.

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.

Does not apply. Nobody is born with C skills.

Competence can be measured objectively. Failure to understand array bounds is prima facie evidence of lack.