Sticking to plain C has the advantage of probably being the subset of C++ (I know, it’s not technically a subset) that’s in widest use.
I prefer using C++ like "C but with cin and cout" and I informally call it C+ :)
Is it not? I thought C++ was explicitly a strict superset of C?
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
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;
More importantly, though - the syntax is not what matters. The languages are very very different in idiomatic use. You just don't write programs the same way with C and with C++. And the gulf between the two only expands with time. A decent C program is almost certainly a poor C++ program, in terms of idioms, utilization of library facilities, and sometimes even in terms of performance (!).