Groovy supports Java >= 9 since versions 2.5.0 / 3.0.0. 

See https://issues.apache.org/jira/browse/GROOVY-8471

So the affected commits to remove Groovy support should be reverted:

https://github.com/qos-ch/logback/commit/047159b662fef2d4019fb2a000ea231db31d12a4

https://github.com/qos-ch/logback/commit/aca78d4bac55a4b5e1c1019e6dc177a786cea985