Asking this question without disqualifying the answer "No breaking changes whatsoever" usually just generates a bunch of different ways of phrasing that.
K&R C no longer compiles. Not that the C to C++14 transition is what I would call an example of greatness, but it is somewhere along the success spectrum.
Seriously, you will someday need to transition your customer base. How do you plan to do it?
foo@virt-ubuntu:~/c$ gcc --version
gcc (Ubuntu 6.3.0-12ubuntu2) 6.3.0 20170406
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
foo@virt-ubuntu:~/c$ cat 'k&r.c'
#include<stdio.h>
int main(argc, argv)
int argc;
char *argv[];
{
puts("hello, world!");
return 0;
}
foo@virt-ubuntu:~/c$ gcc 'k&r.c'
foo@virt-ubuntu:~/c$ ./a.out
hello, world!