Not sure what POSIX falls into. OS-level library?
Java (followed by c#) absorbed many concurrency features into the compiler, by adding language keywords. Is this related to the VM? I think so. Is The library approach still necessary in the presence of VM? Yes. http://stackoverflow.com/questions/30816307/does-java-jvm-use-pthread confirmed that jvm uses pthreads on linux and macOS.
Python, perl and most languages have no VM and use the common add-on “library” approach. The library is usually a wrapper over C threading library.