While it has roots in C, I think it's better to consider it a separate language, especially in more modern forms (Metal is C++14). Metal is locked into Apple's proprietary platforms, but there is other work in compiling C++ to portable shaders, particularly Sean Baxter's Circle, as well as some academic work[1].
There's also of course rust-gpu. I think it's fair to say that in 5 to 10 years we will no longer be writing shaders in C, but will have moved on to a higher level language.