It's more of a fun exercise, I guess. But I do have experience with at least one compiler that doesn't support C99: Zilog's ez80 C compiler. Back in the day I used to program my TI-84+ CE for fun[0], and the only C solution was a pretty bespoke C89-only compiler[1] distributed with a community toolchain[2], which has since switched to clang. It's somewhat irrational, but in the back of my mind it bugs me if the software I write can't run on platforms like that.