With recent changes in Checkstyle 8.21, the API for custom checks was so thoroughly broken, that we can’t immediately adjust. Stay tuned for a new major release.
We will have to wait for a bit until the Checkstyle components maintained by the core team have been adapted (SonarQube integration and SevNTU Checks mostly). Then we can release a new major version (Checkstyle Addons 6.0.0) which will support Checkstyle 8.21 and above.
Also, we wait a little in order to see if maybe the change gets reverted, in which case all would be fine again.
Stay tuned, keep using Checkstyle 8.20 or earlier with Checkstyle Addons, and please bear with us for a little while. Thank you!