Java 9 introduced a major change in the way Java programs are written, because of the Java Platform Module System (JPMS). And that is not something you can ‘skip’ by moving to a higher Java version; upgrading your software to any version after Java 8 means you have to deal with JPMS.
That is the reason a lot of companies are still stuck at Java 8, but at some point in the future they and everyone else will need to upgrade. Java 11 is the first Long Term Support (LTS) version of Java, and that seems to be the version of choice for a lot of migrations.
Upgrading my open source project, JFXtras, to Java 9 a few years back was a breeze. I have had more problems with the build tool (Gradle) than with the modularization of the code. But that is because the code is fairly new and already modularized using Gradle (Maven) artifacts. But in the last weeks I’ve tried to upgrade a 15+ years old custom made ERP system to Java 11… That turned out not to be a breeze.