Another response from Solr devlist: Based on the questions that we've seen over the past month on this list, there are still users with Solr on 6, 7, and 8. I suspect there are still Solr 5 users out there too, although they don't appear to be asking for help - likely they are in set it and forget it mode.
Solr 7 may not be officially deprecated on our site, but it's pretty old at this point and we're not doing any development on it outside of maaaaybe a very high profile security fix. Even then, we might acknowledge it and recommend users update to 8.x anyway. The index files generated by Lucene and consumed by Solr are backwards compatible up to one major version. Some of the API remains compatible, a client issuing simple queries to Solr 5 would probably work fine even against Solr 9 when it comes out eventually. A client doing admin operations will be less certain. I don't know enough about Beam to tell you where on the spectrum your use will fall.
