Most of the really annoying UB is technically in C89 though, it’s just that compilers haven’t really treated it with such contempt for the first two decades or so. I can’t even recall any new kinds of (non-library) UB in C99 or C11 (though there have to be some).
So “old C” in such a case would need to mean “an old C implementation” (or possibly a new one, but simple or configured to behave like an old one), something like GCC 2.8 maybe, and nobody’s using that on desktop. So the language standard version should be mostly immaterial, and it’s not like the C89-to-C17 difference is anything like the yawning C++98-to-C++20 chasm. (This is a carefully phrased statement: C99 had complex numbers, which are annoying, and variable-length arrays, which are a significant change, but C11 demoted both to optional features.)
So “old C” in such a case would need to mean “an old C implementation” (or possibly a new one, but simple or configured to behave like an old one), something like GCC 2.8 maybe, and nobody’s using that on desktop. So the language standard version should be mostly immaterial, and it’s not like the C89-to-C17 difference is anything like the yawning C++98-to-C++20 chasm. (This is a carefully phrased statement: C99 had complex numbers, which are annoying, and variable-length arrays, which are a significant change, but C11 demoted both to optional features.)