I've had lots of old C code refuse to compile on new compilers out-of-the-box, not to mention that you'll likely need to track down old versions of dependencies as well. Sure, you can always get it all to compile with no changes to the code, but the process can be arduous and the result can basically have its own OS's worth in libraries attached to it (not that modern languages are any better here...).
This was a bizarre comment in the OP article, with an even more bizarre reasoning: they "all have versioning mechanisms where opting for new language features will force you off of deprecated APIs"
C code from 1999 will, unless written by a literal psychic, not have "opt[ed] for new language features" that arose in the interim.