This seems to be a common phrasing left over from maybe two or three decades ago. Whenever I see it I assume it's going to be written in terrible non idiomatic c++. Whatever it is, saying c/c++ doesn't not inspire confidence that I'm going to want to learn in that style.

I usually argue for using C/C++ when the intent is talking about C and C++, after all we used to have "The C/C++ Users Journal".

However programming a Redis clone in C/C++ is definitly not something that happens, it is either C or C++, and if it is in the common subset compiled by compilers of both languages, there are definitly better ways to express it.