Thanks to 9.0.1 FP8, we've been able to use Java 8 on Domino for a while, and FP10 makes that support a bit more official at the OSGi level. However, Java 8 is no longer the latest Java runtime, and so anyone writing XPages plugins and compiling/testing them via Maven will likely run into a situation where the compiling JRE is 9 or above. There are a couple changes in these runtimes that add some wrinkles to the process, so I took up the task of working around the problems I hit, and I created a Git repository to contain the code and tips I used to do so:
The README in the repo contains a couple bits of Maven configuration that can be used to successfully compile XPages plugins in Java 9 or 10, and the included project contains a patch fragment for the com.ibm.notes.java.api plugin that serves up the Notes.jar to work around the fact that CORBA has been removed from the standard JRE.
For my needs, those changes made me able to compile a reasonably-complex XPages application, but there may be other edge cases I haven't hit yet. As I do, I'll add information and code there.