There is very little that has changed that isn’t backward compatible, though, and what there is has mostly been in areas that no sensible programmer should ever have relied on in more than toy programs anyway, like inherently unsafe functions or undefined behaviour. You probably have to go back to C99 prohibiting some things that used to be implicit and therefore to code written over two decades ago to find “real” exceptions to that. Compared to many “modern” languages, I’d say that’s still an excellent track record for longevity and compatibility.