I agree that "straight C" is not the best choice anymore... but C++ isn't great either. It inherits most of C's weaknesses and for libraries, it makes it very difficult to use the code from languages other than C++. By making a library in C++ you are basically limiting it to being used only in C++ projects. (Yes, wrapper generators for C++-to-whatever exist, but my experience with using e.g. Qt from other languages has always been pretty terrible).
Even in 2019, the C ABI is the one universally agreed-upon cross-language ABI for languages compiled to native code.
Even in 2019, the C ABI is the one universally agreed-upon cross-language ABI for languages compiled to native code.