The name of is_Worker_thread suggests it might have something to do with GC work gangs, but it doesn't. Rather, it is true for NonJavaThread.

The only code that uses is_Worker_thread is Shenandoah. and only in asserts. Some of those asserts seem to be confused because of the name, and are clearly tautological, e.g. asserting a thread is either a JavaThread or a Worker (i.e. NonJavaThread).