Just because I'm using a C++17 compiler doesn't mean I'm happy with the standard library.
I still have to google every time I need to sleep the current thread ; and don't get me started on iostream!
However, I'm happy to be able to return a buffer without copying it (move semantics), to precompute frequency tables at compile time (constexpr), to be able to pass around functions without having to create a dedicated class (lambdas), not to have to worry about having an extra conversion (auto) ... And soon, I will be happy to write serialization code without relying on dirty tricks (introspection).
Given that most new C++ features in the "modern day" are implemented as std::whatever, in the "everything is a library" way, it's extremely relevant.