[janoc]

It is not and never was.

Look at for ex. what const means in each, or auto. Or variable length arrays (feature that doesn't exit in C++). Or designated initializers that C++ didn't have for a long time.

Here is a good list of differences: https://mcla.ug/blog/cpp-is-not-a-superset-of-c.html

[gmueckl]

C++ is almost a perfect superset of old C before C99. There are odd corner cases where prograns are parsed differently, but these are quite contrived. The 3rd edition of The C++ Programming language lists a few cases. Since then, C has had may additions to the language that made it diverge from C++. Your linked article is almost entirely about these new C additions.

[janoc]

Even for the old C it wasn't a superset.

There are plenty of legal C programs that aren't legal C++ - if for nothing else, then because C++ has more reserved keywords which are perfectly legal identifier names in C.

E.g. this is legal C but not legal C++:

int template = 10;