With the release of 2.0, some methods are deprecated, some of them without implementation, for example https://git.magnolia-cms.com/projects/MODULES/repos/rest-client/browse/magnolia-rest-client/src/main/java/info/magnolia/rest/client/registry/RestClientRegistry.java#109

Make a list of all deprecated methods, and then make sure all have a correct implementation, don't leave deprecated methods without implementation.