Between JDK8 and 9 there was the introduction of the module system, which broke many systems, cause they did somethings they weren't allowed to do in the first

Between JDK8 and 9 there was the introduction of the module system, which broke many systems, cause they did somethings they weren't allowed to do in the first place (and which wasn't included in the backwards compatibility guarantees): Access internal JDK classes. There were some compromises to make it still work (e.g. they continued to allow access to some parts of sun.misc.unsafe until there's a replacement with the same performance, cause most of the things in there are pretty fast), but some things just didn't work with the module system and had to be changed. By now, I'd say if you depend on a library that still doesn't work on JDK9+ you should replace it, but depending on your requirements that may not be so easy and so some people are still stuck on JDK8.