The enableVirtualThreads setting was added in #7134 to "Support Project Loom in Jetty 10 & JDK 17+". The documentation for this setting says it "enables virtual threads for Jetty’s thread pool." Setting that flag to true simply causes Jetty's QueuedThreadPool to be backed by a virtual thread executor. This not only causes the virtual threads to be pooled, which is an anti-pattern, but also bypasses Jetty's built-in support for virtual threads. Instead, Jetty recommends creating the thread pool as normal and setting a virtual thread executor via QueuedThreadPool.setVirtualThreadsExecutor().

I think AbstractServerFactory should be reworked to configure Jetty as described when this flag is set. I'm not aware of any use-case where the current behavior would be desired.