Virtual threads do not do it voluntarily. They have no knowledge or control over where they might yield. Without forced preemption, I guess you can say that as long as they don't call into the JDK in any way (including e.g. throwing exceptions) or any third-party library then they shouldn't normally expect to yield, but I don't count calling any code you haven't personally written "voluntarily yielding".

We call them preemptive with or without forced preemption -- in line, I think, with the definitions on Wikipedia -- but whatever you choose call them, the concurrency programming style is the same as that with threads today or Go's goroutines, and is different from the style of C#/JS's async/await, Kotlin's coroutines, or more explicit async code, all of which result in user code relying on knowing where yield points (possibly) are (i.e. "critical section" by default). BTW, even with OS threads, when you run transaction-handling code, as opposed to long-running computation, time-sharing preemption is the exception rather than the rule.