Elasticsearch 6.0 introduces strict content-type checking for REST requests, a shift from previous versions where the content-type was optional and the server would attempt to guess the appropriate type if it was missing or unrecognized. This change is aimed at enhancing clarity and security by ensuring that each request explicitly states its content type, thus reducing unexpected errors and protecting against Cross Site Request Forgery (CSRF) attacks. While this adjustment requires users to include the correct content-type for request bodies, it aligns with Elasticsearch's commitment to providing more predictable and reliable operations. The update may necessitate adjustments for those using third-party clients or command-line tools like curl, but it is seen as a valuable step in bolstering the security and consistency of Elasticsearch's functionality.