[pjmlp]

I am not claiming, it is a fact.

It was dropped from C11, moved into optional annexes and all commercial compilers that were yet to be fully C99 compliant just moved to C11 instead, without bothering with optional annexes.

The world of C compilers isn't only GCC and clang.

[Chris_Newton]

Something being dropped is not the same as making something optional. In this case, the latter action was an explicit recognition of the reality that many compilers never fully implemented the previous standard anyway.

The point here was about backward compatibility. What standard C99 code is not also standard C11 code? What used to compile a few years ago but won’t compile today? We all understand that there are some minor exceptions, but I think it’s fair to say the answer to both question is not much.

[pjmlp]

VLAs, can't make it any clearer than that.

Any use of Annex K functions, gets(), any code that is considered semantic invalid under C11 memory model, or gets wiped away with the new UB cases from C17.