Per the RfC a while back[1] and the previous announcement,[2] we're about to remove the deprecated dbg, txt, and yaml output formats from the API. This is planned for 1.27.0-wmf.8, which is currently scheduled to be deployed to WMF wikis starting December 1, 2015.[3]
Users using the yaml format should be able to easily change to json, as the format is entirely identical.