No, virtual thread stacks are not roots! This is one of the main design highlights of the current Loom implementation. In fact, at least currently, the VM doesn't maintain any list of virtual threads at all. They are just Java objects, but the GC does treat them specially.
Unlike any other object, the location of references on a stack can change dynamically, so the GC needs to recognise those objects and walk their references differently. There are other subtleties, too.
