The GET /orderables should support If-Modified-Since HTTP header for all of the incoming requests. If the header is present, the request should only return a whole representation if the server modification date of ANY of the given orderables resources is latter than the date provided in the If-Modified-Since request header https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since if the dates are the same or the version in request header is latter than the server version of all resources, the request will return response code 304 Not Modified and the response body will be empty. Response header Last-Modified should contain the latest modified date of all found resources.
If-Modified-Since request header is supported in GET /orderables endpoint but is optional
If the header is not provided, the endpoint returns the response as it used to
If the header is provided, the request returns a 200 response with the body only if the server version of ANY of the found resources is latter by comparing the modification dates in the header and the modified date in server resource
If the header is provided, but the header version is the same or latter than all of the server modification dates for found orderables, the endpoint returns 304 Not Modified and doesn't attach the request body
Last-Modified header should always be present