Restlet changelog says that some extension will be removed with the next minor release. I could not find any information if the extension will be removed completely or reimplemented.
If the extension is no longer supported I have to search for another solution.
May somebody have more information about the current situation.
Edit: Posted to: https://github.com/restlet/restlet-framework-java/issues/1375
There is now an official statement that the OAuth module will not be replaced but permanently deleted.
https://github.com/restlet/restlet-framework-java/issues/1375#issuecomment-807236231