Going against the flow I would say it depends on the context.
The term "C/C++" is usually not appropriate when saying something like "this is a C/C++ program", but this has been explored to depth in other answers.
However, there might be contexts where C/C++ can be appropriate.
- There are various libraries which usually have both a C and a C++ API. I guess it's not far from the truth if you call such a thing a C/C++ library. We humans like to compress information, so saying "opencv is a C/C++ library" is short, clear, and understandable, compare it to saying "opencv is a library, which is shipped with headers both for C and C++".
- You can talk about language design and syntax. From the point of view of the language syntax, you might say that a language has C/C++ -like syntax.
- You organize a coding contest, and you accept both solutions written in C and in C++
- You are hiring a new programmer, and most of the tasks will be either C or C++, so the programmer is expected to know both languages. It's common in embedded development, where C is more suitable for some (usually very small) microcontrollers, and C++ for others. In this case you might say you are looking for a C/C++ programmer.