Hi @ChocoPowwwa, officially no, the sink does not support Elasticsearch 2.3. By us it has only been tested on Elasticsearch 1.4-1.5.
With the HTTP client it should be possible to send events to all 1.x Elasticsearch clusters. I have tested it briefly with a 2.3.4 cluster but not extensively and not in a production environment - it was able to successfully sink events to the cluster however.
So please do test the sink with a 2.3.4 cluster and let us know if any issues crop up!
The other solution to get the sink working with a 2.x cluster would be to build the sink manually with this dependency bumped to 2.0.3 rather than 1.0.3. See the Jest Github for more version details.
Hope this helps,