For more recent versions of Java, Oracle has removed some of the "less popular" features in an effort to slim down the Java download size. One of those "less popular" features is the most modern graphics toolkit in Java, which I'd started to migrate to for features such as reorderable tech and unit lists, filterable tech and unit lists, downloading units from CFC directly to a scenario folder, and Help pages that include images of what they're describing. These features range from significantly more difficult to impossible to implement with the old graphics toolkit that is still in newer versions of Java.
Oracle is also switching to discouraging non-developers from downloading Java directly; the tradeoff of that is that in the Java 8 world, you would download Java once (from the link above), for a download of 60 MB, and each version of the editor would be about 2 MB; in the new world, you'd simply download the new version of the editor - but as that would include Java, it would be somewhere on the range of 40 - 60 MB each time. For enterprise software (Java's strongest market), the size of the enterprise software would dwarf the size of re-downloading Java each time, but that is not so much the case for small software such as the editor. This also switches the distribution model from building it once (i.e. on my Windows 8.1 desktop) and having it run anywhere, versus having to do three separate builds for Linux, Windows, and Mac (quite possibly on different physical machines), and uploading each of them (as I'd have to include the Linux/Windows/Mac version of Java with the editor). In other words, the overhead for distributing new versions becomes significantly higher. Similar overhead issues are why the "Legacy", "Windows XP", and "Windows 95" editions of the editor are not updated alongside every main editor update.
It is possible to re-bundle the new graphical toolkit with software running newer versions of Java, but it's still subject to the new-version overhead mentioned in the previous paragraph, and there are additional complexities if you want to have software run with both Java 11 (the "new" version I could target) and Java 8. Dropping Java 8 would mean dropping some older operating systems as well, such as Windows XP/Vista, and OS X 10.8 through 10.somewhere around 11.
In 2017 I thought this would all cause significant havoc, but as it turns out you can still download Java 8 and run it from the link above, and it will not try to auto-upgrade itself to a version that won't run the editor. Thus, rather than go through the horse and pony show of trying to move everything to Oracle's new model, I've preferred to spend the (relatively small) amount of time I spend on the editor on new capabilities. If at some point Java 8 really does become unavailable or support ends (and in some good news, multiple companies other than Oracle have stated they intend to provide security updates for Java 8 into the mid-2020s), I'll likely have to change that.
(As an analogy to another language, Oracle dropping not only this graphics toolkit, but also several "less popular" enterprise components in Java 9 and Java 10 is kind of like how Python made some "relatively minor" backwards-incompatible changes from Python 2 to Python 3, and a decade later a lot of companies and programs are still using Python 2)