C89 is too old, too many grammar restrictions. And now the mainstream compiler basically supports c99.
The versions do have other differences, of course, but they're rarely used (the next most common one is designated initializers, which for me are the most annoying to convert).
I mean, it is your code, i just found the grammar restrictions part weird since really there isn't that much of a practical difference between C99 and C89 to the point i personally never felt the need to use C99.
(note that i focus on the language bits, C99 provides some extra library functionality but even then the most useful part is stdint.h which can be implemented in almost any C89 compiler anyway)
printf("Hello World!\n");
int x = 7;
printf("%d!\n", x);
return 0;
Of course that is my opinion (and i'm generalizing of course), but that comes from my experience with working with others' code.
Unless a function has only two lines of code.