Currently RestClientBuilderImpl does not take ProxySelector into consideration.
Only system-property "http.proxyHost" and proprietary config "org.jboss.resteasy.jaxrs.client.proxy.host" .
The ProxySelector has been added by Ivo Colombo in the, now archived, smallrye-rest-client.
see here: https://github.com/smallrye/smallrye-rest-client/blob/master/implementation/src/main/java/io/smallrye/restclient/RestClientBuilderImpl.java#L159
It would be nice to have this feature again in the Resteasy variant.