Revapi has been written with extensibility in mind. Everything from API scanning, filtering to report generation is extensible. Revapi comes with a couple of extensions that make it useful without a need to write any code yourself (even though writing an extension is super simple and fun ;) ).
Revapi comes with a couple of extensions that are bundled by default with the maven plugin, ant task and the CLI. These include the extensions contained in the Revapi Basic Features and of course the Revapi Java extension.
There are also some 3rd party extensions, for example https://github.com/jenkinsci/lib-revapi-hpi-extractor that the Jenkins CI uses to make Revapi understand the HPI packaging of the Jenkins plugins or https://github.com/mulesoft/mule-revapi-extension used for analyzing Mule modules.