Legacy `java.lang.ThreadGroup` has been degraded in this release. It is no longer possible to explicitly destroy a thread group. In its place, `ThreadGroup` is changed to no longer keep a strong reference to subgroups. A thread group is thus eligible to be GC'ed when there are no live threads in the group and nothing else is keeping the thread group alive.

The behavior of several methods, deprecated for removal in prior releases, are changed as follows:

- The `destroy` method does nothing.

- The `isDestroyed` method returns false.

- The `setDaemon` and `isDaemon` methods set/get a daemon status that is not used for anything.

- The `suspend`, `resume`, and `stop` methods throw `UnsupportedOperationException`.

For further details, see the [JEP 425, section java.lang.ThreadGroup](https://openjdk.java.net/jeps/425#java-lang-ThreadGroup).