As of release 1.2.9 support for logback.groovy has been entirely removed from the code base.

Ref. https://github.com/qos-ch/logback/commit/3b9cd0efe0b3390026a04a6092aa03e433ddd330#diff-03f8693b683877937c953191b330f1c0384c020ec6fc60717bcbd29572e15e05

It should be reflected in the documentation that existing logback.groovy implementations have to be rewritten into logback.xml.

Documentation: https://logback.qos.ch/manual/groovy.html

Or, Groovy support should be reintroduced