But the issues Fil-C prevents, it prevents in very old programs compiled with it , if they compile, just as much as current programs.

At compile time, certain patterns that are accepted in standard C are rejected by Fil-C, and the source code has to be changed if you still want to compile the program. At run time, Fil-C prevents memory-related security issues by reliably detecting invalid memory reads and writes that are usually part of an attack and producing deterministic behaviour, sometimes terminating the program, instead of letting the attacker take advantage of undefined behaviour that happens to compile to something the attacker can use. Both of those features work just as well with very old C programs as current ones.