The C preprocessor is hilariously underspecified in the standard, so implementing the standard doesn't guarantee that you'll be able to handle real-world C programs (even ones that don't use GNU or clang extensions).
K&R preprocessor was indeed underspecified and allowed lots of variations---much of those issues can be seen in the GCC manual [1]---, but the current ISO C is much better at that job AFAIK. I think `## __VA_ARGS__` is the only popular preprocessor extension [2] at this moment, as the standard replacement (`__VA_OPT__`) is still very new.
[1] https://gcc.gnu.org/onlinedocs/gcc/Incompatibilities.html
[2] Assuming that we don't count things like `#pragma` or `#include_next`, which can be added without affecting other preprocessing jobs.