> the language itself

The "language" is conventionally thought of as the sum of the effects given by the { compiler + runtime libraries }. The "language" often specifies features that are implemented exclusively in target libraries, for example. You're correct to say that they're not "language features" but the two domains share a single label like "C++20" / "C11" - so unless you're designing the toolchain it's not as significant a difference.

We're down to ~three compilers: gcc, clang, MSVC and three corresponding C++ libraries.