The C ISO document only ships about once or twice per decade, so being a "15 year old standard" means there are in fact only two newer standards C17 and C23.

C17 is largely bugfixes. An "upgrade" to C17 probably wouldn't even change most of the whole files in Linux, let alone most of the lines of code, it's just not relevant. C23 is a larger change but it's also very young, much younger for example than the Rust for Linux project.