Seems it's broken since JENKINS-24243 where proxy settings where moved from RemoteRepository to MavenRepositorySystemSession.

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 

/**
 * Sets the proxy selector to use for repositories discovered in artifact descriptors. 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.
 * 
 * @param proxySelector The proxy selector to use, may be {@code null}.
 * @return This session for chaining, never {@code null}.
 * @see org.sonatype.aether.repository.RemoteRepository#getProxy()
 */
public DefaultRepositorySystemSession setProxySelector( ProxySelector proxySelector )

^^ 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.