[pron]

Virtual threads are already available in the current version of the JDK (19) as a Preview feature. Because it's not a preview language feature but a preview API, libraries can use it without compiling with --enable-preview, either with reflection or by having the application supply a virtual thread ThreadFactory, which means they can make use of virtual threads if the application turns preview features on.

Preview language features are different as they require compiling with --enable-preview, which creates a "poisoned" class file that cannot be loaded at all without preview enabled, so preview language features are not recommended for use by libraries, but preview APIs are fine (see JEP 12: Preview Features https://openjdk.org/jeps/12).