C++ is C compatible so more than 50 years of backward compatibility. Even today the vast majority of C programs can be compiled as C++ and they just work. Often such programs run faster because C++ a few additions that the compiler can use to optimize better, in practice C programs generally mean the stronger rules anyway (but of course when they don't the program is wrong).
<pedantry corner>CFront was never compatible with K&R C to the best of my knowledge, so the actual start date would be whenever C89-style code in widespread use; I'm not sure how long before 1989 that was.
I can tell that during 1999 - 2003, the aC compiler we had installed on our HP-UX 11 development servers, still had issues with C89, we had #defines to use K&R C function declarations when coding on that system.
Kind of, compatible with C89 as language, and with C23 to the extent of library functions that can be written in that subset, or with C++ features.
And yes, being a "Typescript for C" born at the same place as UNIX and C, is both what fostered its adoption, among compiler and OS vendors, and also what brings some pains trying to herd cats to write safe code.
