Seems it's broken since
where proxy settings where moved from RemoteRepository to MavenRepositorySystemSession. JENKINS-24243
The proxy set into MavenRepositorySystemSession never gets propageted to RemoteRepository now since the plugin does not use RemoteRepositoryManager nor sets the proxy manually anymore.
This results in all the calls to repositorySystem.resolveVersionRange, repositorySystem.deploy, repositorySystem.resolveDependencies etc. performed without proxy
^^ Note that this selector is not used for remote repositories which are passed as request parameters to the repository system, those repositories are supposed to have their proxy (if any) already set.