Comment by Joker_vD

Uh, are they mandatory again?

                       For these reasons, we propose to make variably-modified types
    mandatory in C23. VLAs with automatic storage duration remain an optional language
    feature due to their higher implementation overhead and security concerns on some
    implementations (i.e. when allocated on the stack and not using stack probing).

from N2778 (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2778.pdf)