The reason the community is struggling with Java 8 to Java 9 is that 9 broke a TON of stuff. It's not backwards compatible at all. Basically every non-trivial app broke. The breakages are mostly in things that were technically never guaranteed, even trivial things like the version number went from "1.8" to "9" which makes sense but broke loads of programs that tried to parse the version.
But some of it is also just bad processes and habits in the community. Some of the programs that broke due to the version number change from 8 to 9 broke again from 9 to 10 just months later because they had enums that listed every possible version.... their codebase literally encoded the assumption that new major java releases hardly ever happened!
I heard one good approach is to start testing newer releases in your CI systems as soon as possible but wait for the next LTS ever few years to migrate production.
Apache Groovy hasn't even updated its syntax to support Java 8 lambdas. It's coming in Groovy 3, apparently.
If Groovy's project managers at Apache don't want a Groovy 3 release to avoid a debacle like Python 3 or Grails 3, then they should just come out and say so. Instead, they're letting others waste their time working on it while putting up roadblock after roadblock to slow it down.