Where can I find documentation on JAVA_OPTS / JRE flags supported by Zulu
I've looked here but cannot find anything https://docs.azul.com/zulu/zuludocs
Zulu is a build of the OpenJDK, so the flags on the command line are those for HotSpot, which is the Virtual Machine implementation.
Probably the best place to go for command-line flags is Foojay.io. For example, the JDK 8 flags are listed here:
https://foojay.io/command-line-arguments/openjdk-8/?tab=alloptions
Other versions can be found under the OpenJDK Command Line Arguments section on the main page.