I would like to point out also that C++ is not yet compatible with itself. Even GCC has dozens of C++ features it doesn't implement and it was the first compiler to implement 100% of C++11: https://gcc.gnu.org/projects/cxx-status.html
Technically true but in the real world those are rare edge cases.
Only because they're not used. If you're not going to implement the spec then is it really a spec?
It could also be argued that they aren't used because not all major compilers implement them.
If there was demand major compilers would implement them. That is why despite being difficult all major compilers are working on getting modules working.