There are obviously many very old Solr installs out there. People are still reporting issues for Solr 4.x and 5.x. This is a proposal that Solr will print a warning in the logs and display a warning in the Admin UI Dashboard when running a (probably) outdated version.
I do not aim to "call home" to Apache to check versions, but instead parse release date from 'solr-impl-version' string and warn if more than 1 year old. That should be very conservative as there will almost certainly be a handful new releases, with potential security fixes.