Elasticsearch 2.4 officially reached its end of life in february 2018, and Elasticsearch 5.2 on 2018-06-08, according to this document: https://www.elastic.co/support/eol

The Elasticsearch integration in Search 5 was experimental, so dropping such support for outdated versions of Elasticsearch is not outrageously unexpected.

Also, users migrating to Search 6 will be able to first migrate do ES 5.6, then upgrade to Search 6.

See also HSEARCH-3262 Open