Pedantically, yes VLAs are a mandatory part of C99 (only). Practically, there has been some resistance so they were reverted to optional in later standards (in C11/17, the whole thing is optional; in C23, variably modified array types are mandatory but the ability to allocate arrays of such types on the stack is not). In any case, MSVC is quite a bad benchmark as far as C (not C++) standards conformance goes—it’s been quite some time since Sutter’s (in)famous post[1] and things have improved, but not to the point that I’d believe C to be a priority for Microsoft.

(Note MSVC has alloca—and Microsoft’s own libraries in the past have done unwise things with it—so the safety argument for the lack of support does not fly.

[1] https://herbsutter.com/2012/05/03/reader-qa-what-about-vc-an...