The point was that the way VLAs were designed in C99, it was a motorway to stack corruption, leading to dropping it out as optional feature in C11 and C++ not even considering them.
I don't see anything in the wording of C99 around VLAs that requires them to always be allocated on the stack. That implementations did so regardless is, arguably, a defect on their part, and a quality-of-implementation issue.
Well, the whole point of VLAs was supposed to placate the math/numeric crowd somewhat, and they generally expect things to be a little bit higher-level than what's otherwise mandated by the C "zero overhead" philosophy. Since VLAs are opt-in, code that cares more about perf could always skip them and not pay the tax.
But yes, it was probably too naive to expect that to work out.
