Github user tzulitai commented on the issue:
https://github.com/apache/flink/pull/6043
The main reason why the discussion leaned towards the current proposed
change by this PR, was that only Elasticsearch 5.6+ supports REST.
Only working towards a clean-cut module that uses REST, would mean that we
still wouldn't be able to support Elasticsearch 5.2+ up to Elasticsearch 5.5.---
