As of the latest release (R2023b), most of what I posted over the years on the undocumentedmatlab.com website (and in the 2011 book) is still relevant and works just as it had a decade ago, including the undocumented reliance on (and customizations of) the legacy Java-based GUI. The new GUI functionality that has come into focus in recent years (uifigure, App Designer etc.) is based on non-Java technology (HTML & JavaScript). But the legacy Java-based GUI is still functional alongside the HTML/JS-based GUI, at least for now.
MathWorks invested most of its R&D efforts in recent years on the non-Java functionalities, so the Java-based Matlab code has remained largely unchanged so far. My guess is that perhaps 90% of the contents of the 2011 book is still relevant in R2023b, certainly the major concepts. Considering the fact that the undocumented aspects were always under the risk of being "removed without prior notice in a future Matlab relase", the fact that so many of them remained useful for so long (close to 2 decades in some cases) is absolutely amazing in my eyes. It's been a [very] long run, but nothing lasts forever.
Going forward, MathWorks will eventually stop supporting Java-based GUI to some degree (when and to which degree is not publicly known at this time). Note that this phase-out of Java was promised/warned-about for many years already (since 2016 I think), who knows when it will ever happen. Probably not more than a few years though. At that time, it is still unclear whether Java-based GUI customizations will break completely (run-time errors) or exhibit some sort of graceful degradation. AFAIK, MathWorks have so far not made any public comment about either the expected timeline or the behavior. I believe this vagueness is counter-productive, but they have their reasons.
In any case, even if and when the Java phase-out occurs, non-GUI Java-based functionality are expected to still work by loading a user-supplied JRE. As long as you specify a supported JRE and properly configure Matlab to use it, you should be able to keep using non-GUI Java in your Matlab programs, and probably also pure-Java GUI. Note that the interface to using external Java libs is fully documented just like the external interface to Python, C# etc. - there's no undocumented aspect here.
Also note that there are many undocumented aspects of Matlab that are not Java-based - these will probably continue to be relevant even once Java GUI is gone. These include a myriad number of hidden properties and methods of otherwise-documented Matlab objects.
All that being said, and in light of the eventual expected Java phase-out, I do not plan any new edition of my 2011 book "Undocumented Secrets of Matlab-Java Programming". In contrast, I do plan a 2nd edition of my 2015 book "Accelerating Matlab Performance", which is long overdue. But then again, so is MathWorks with its Java phase-out, so I think I'm in good company :-)
