There are two Java Rest clients named Java High Level REST Client and Java Low Level REST Client to integrate with Elastic Search.
When did Java High Level REST Client got introduced in Elastic search and why?
If you follow the official documentation you will get to know the differences, how they work and the reason behind adding the Java High Level REST Client(JHLRC).
Coming to the first question when JHLRC was added, as shown in official doc, it was added in 6.0.0-beta1 release.
Coming to the second question,
JHLRC works on top of JLLRC and provides additional functionality like marshaling and unmarshalling of requests, which low-level client doesn't provide, apart from that it provides both sync and async versions of APIs.
More info on the low-level client can be found here and about high-level client can be found here, which also explains the internals and how they work.