People are probably more likely to fix their code if they're told how -- it would be useful to have

void new_function() { }
void __attribute__((deprecated("use new_function instead")) old_function() { }

int main(int argc, char **argv) {
        old_function();
}

resulting in
test.cpp: 6: warning: 'old_function' is deprecated (declared at test.cpp:2, use new_function instead)