Plugin settings UI points to the codehaus docs

[romnempire]

Using plugin version

Groovy-Eclipse Feature 2.9.2.xx-201508121448-e45 org.codehaus.groovy.eclipse.feature.feature.group Codehaus.org

If you don't install any extra compiler features, Preferences > Groovy > Compiler generates a link explaining how to add extra compiler features, pointing at
http://docs.codehaus.org/display/GROOVY/Compiler+Switching+within+Groovy-Eclipse which doesn't exist anymore. Ergo, there is no way to learn to add extra compiler features without archive.org-fu.