Since introducing JSON in V3 of the API we have seen an overwhelming adoption of the new format. We have investigated the usage of XML in the ACCEPT header, and we have seen that the usage is now less than 1% of the total, with 99% having already switched to JSON.
Supporting XML requires a significant portion of our development capacity – resources that we would rather spend on creating new features and improving the API. We have therefore decided to drop our support for XML starting with Retailer API V5, which will launch on 1 April. We will continue to support XML for V3 and V4, but you should plan to migrate to JSON as soon as possible since older versions will be removed in accordance with our release plan.