includevirtualthreads=y is a debug agent option that forces the debug agent to keep track of all created virtual threads, and return them when JDI vm.allClasses() is called. Some tests require this functionality in order to discover debuggee threads. However, always using it eliminates includevirtualthreads=n testing coverage, which is more important since IDEs normally don't run with includevirtualthreads=y. Furthermore, it will negate the changes being planned for JDK-8282441 since they will be disabled when using includevirtualthreads=y. Thus we won't have sufficient testing for testing JDK-8282441 changes.

It turns out there is only one com/sun/jdi test that requires includevirtualthreads=y, and it is pretty easy to just make that one test run with includevirtualthreads=y rather than every test.