Broken or ancient compilers. That's as much C's fault as it's Intel's fault that Windows 10 doesn't run on Pentium MMX. Implicit functions are not part of C; they were gotten rid of back in 1999!
I have to disagree about implicit functions not being a part of C anymore. I've yet to see one generate an error; this is optional, and frequently not used. I'm including brand new, state of the art compilers, even (default) gcc. Of course, not even emitting a warning is bananas.
I confirmed this, with the following snippet:
// start fail.c
int fail = 0;
// end fail.c
int main(void)
{
fail();
return 0;gcc --std=c99 -g main.c fail.c -o main.exe && gdb main.exe -ex run
# Cmd line junk, however a warning was emitted for implicit function "fail"
Program received signal SIGSEGV, Segmentation fault.
0x00407020 in fail ()
This is not really a debatable matter. There's an international standard for what is, and what is not a part of the C language.
Errors as you seem to understand them are optional for everything else except for the #error preprocessor directive. For all the other invalid C programs, only a diagnostic (like the warning you got) is required, and a conforming implementation is free to complete translating the invalid translation unit. I don't see why that would be an issue, as it's very easy to switch those warnings into translation errors if wanted.