When I tried debugging and using the groovy script extension, I went to GitHub to look at the source and found this file:
https://github.com/jenkinsci/artifactdeployer-plugin/blob/master/src/main/resources/org/jenkinsci/plugins/artifactdeployer/ArtifactDeployerPublisher/help-deletedRemoteArtifactsByScript.html

I never found this displayed in any Jenkins configuration dialog.
Maybe it is missing an inclusion somewhere?