Apache Solr 6.0.0 is available since April 2016. Currently Apache Solr claims to support the previous major versions with bug and security fixes (http://lucene.apache.org/solr/community.html). Following that scheme Solr 4.x which we use in DSpace is EOL.
Solr 4 was the last version of solr that supported the integration as war. Every version of solr after 5.0.0 is provided as standalone application. That means we probably won't be able to ship solr as a DSpace module anymore, but have to include it into the list of requirements.
I'm not sure if we should upgrade SOLR in DSpace 7.0 only or if we should backport an upgrade to older versions of DSpace too. While we ship solr with a filter that restricts access to solr to localhost, solr will always parse search and browse queries which are influenced by DSpace's enduser.