The RepositoryPublishURLSerializer was removed from the plugin API.
Distributions are now Master/Detail. All plugins will require updating to provide a detail Distribution. Here is an example of pulp_file introducing the FileDistribution as an example of changes to match.
Publications are now Master/Detail. Plugins that use Publications will need to provide a detail Publication. Here is an example of pulp_file introducing the FilePublisher as an example of changes to match along with its follow-on change.