The Apache Camel-based Elasticsearch forwarder feature uses the ES Transport API internally to send messages. This is not ideal because the transport API is not intended to be compatible across major versions. This also creates an undesirable dependency between the version of Camel that we use and the version of Elasticsearch that we can support.
From the ES docs re: the transport protocol:
The client must have the same major version (e.g. 2.x, or 5.x) as the nodes in the cluster. Clients may connect to clusters which have a different minor version (e.g. 2.3.x) but it is possible that new functionality may not be supported. Ideally, the client should have the same version as the cluster.
To support multiple versions of Elasticsearch, we have an alternative REST-based forwarder now so the transport-based version should be deprecated.