C23 on the other hand landed a bunch of hard changes, and "it didn't break my code" only matches the reality most people observe with Rust too. The changes you mention didn't break my Rust either.

But some trivial C did break because C23 is a new language

https://c.godbolt.org/z/n9vhMGYW5

In fact those Rust changes aren't language changes unlike C23, the first you linked is a choice for the compiler to improve on layout it didn't guarantee, anybody who was relying on that layout (and there were a few) was never promised this wouldn't change, next version, next week or indeed the next time they compiled the same code. You can even ask Rust's compiler to help light a fire under you on this by arbitrarily changing some layout between builds, so that stuff which depends on unpromised layout breaks earlier, reminding you to actually tell Rust e.g. repr(C) "I need the same layout guarantees for my type T as I'd get in C" or repr(transparent) "I need to ensure my type wrapper T has the same layout as the type I'm wrapping"

The second isn't a language change at all, it's a change to a library feature, so now we're down to "C isn't stable until the C library is forever unchanging" which hopefully you recognise as entirely silly. Needless to say nobody does that.

I'm trying to drag one program at $employer up to C99 (plus C11 _Generic), so I can then subsequently drag it to the bits of C23 which GCC 13 supports.

This all takes times, and having to convince colleagues during code reviews.

What C23 has done is authorise some of the extensions which GCC has had for some time as legitimate things (typeof, etc).

However the ability to adopt is also limited by what third party linters in use at $employer may also support.