What's the reason for that and what can be done?
Well, Apache NetBeans (incubating) 9.0 is specifically focused on Java SE only. Why? Because NetBeans is extremely large (it will be by far the largest project in Apache, once everything has been donated), which is because it has existed for over 20 years and provides support for a very wide range of technologies. Since so many files all needed to be audited before they could be donated to Apache, the decision was made to donate NetBeans in pieces, i.e., not everything at the same time because that would have taken much too long.
And since NetBeans is modular, doing an incremental donation was not difficult to architect. The first donation focused specifically on the underlying core, i.e., the NetBeans Platform (e.g., the module system, window system, menubar, etc etc) and, to enable the result of the first donation to be usable for general users and not just NetBeans Platform developers, the various Java SE features were included too, e.g., Java project templates, Java editor, and new Java features such as support for Jigsaw, JLink, and JShell.
So, does that mean that all the features that are not integrated into Apache NetBeans (incubating) 9.0 cannot be used? No, it does not. Go to Tools | Plugins in Apache NetBeans (incubating) 9.0 and, in the Settings tab, register the NetBeans IDE 8.2 update center:
Though note that none of these features have been tested with Apache NetBeans (incubating) 9.0, they should work, in most cases. If not, don't panic. Just sign up to the Apache NetBeans mailing lists, as explained at netbeans.apache.org, and start a new thread where you'll highlight some problem you've encountered, after which someone will help you or you'll be advised to create a new issue in Apache NetBeans Issuezilla.
Where is all this documented, how can anyone know what will be donated in which order, etc? Easy, go here, to the end of this page: