I haven't read the article, but following this effort from the compiler side, I do not think this is the case. Also there is exactly one ISO C standard. Newer versions supersede older ones from a legal point of view. The current one is ISO C17 which will soon be replaced by ISO C23 (officially next year). Older C standards are historical documents and code targeting them is not conforming to ISO C. The C committee takes extreme care not to break existing code except for very good reasons. So code that can only be compiled assuming older versions of ISO C will likely use questionable constructs which were removed in later versions for good reasons.