SolrJ's HttpSolrClient ignores the HTTP proxy configuration from system properties http.proxyHost and http.proxyPort. This used to work with Solr 6.6.5.
Solr 6.6.5 used org.apache.http.impl.client.SystemDefaultHttpClient under the hood, which took system properties for HTTP proxy config into account. The deprecated SystemDefaultHttpClient class was replaced as part of
SOLR-4509. SolrJ now uses org.apache.http.impl.client.HttpClientBuilder#create to create an HttpClient, but it does not call #useSystemProperties on the builder. Because of that, the proxy configuration from system properties is ignored.