We want to surface deprecation warnings on the client side more visibly, as today they come back as headers which might not be being examined by an application developer using the Java low-level REST client. This issue proposes that we add a strict deprecation mode to the low-level REST client with the following properties:

• if strict mode is off (default for 6.x), we log all deprecation warning headers
• if strict mode is on, a deprecation warning header in the response fails the request
• we can consider in the future changing the default to strict mode being enabled