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.

It was a resounding endorsement in 2020, but the proof is in the pudding. So far I am not impressed. It is also not something I think is terribly important. Even if they managed to improve memory safety in the kernel using Rust (but I assume improvements in C tooling will have a much bigger impact), the reality is that for the overall Linux community (but maybe not Google and co.) others things would be far more important (such as a proper responsible disclosure process, the recent failures were pretty bad).