Elastic has also closed source the Java clients, as you can see from Mvn: Maven Repository: org.elasticsearch.client » elasticsearch-rest-high-level-client » 7.13.3 (mvnrepository.com) We should think about building similar clients which are Apache 2.0 licensed.
This is an issue for any software which is consuming this as they in turn have issues with license compliance. This was brought to my attention by Wu-Sheng who is the creator of Apache Skywalking. They are using the Elastic Client in the product, and hence this is a problem for them since Apache projects are required to have Apache 2.0 code only. The same can be said for CNCF projects.