Per the RfC a while back,[1] we've now removed the wddx and dump output formats from the API. This is expected to be a very low-impact change, as logs don't show anyone actually using these anymore.

This change should be deployed to WMF wikis with 1.26wmf13, see https://www.mediawiki.org/wiki/MediaWiki_1.26/Roadmap for the schedule.

Note that the yaml, txt, and dbg output formats are scheduled for removal in November 2015. Users using yaml should be able to easily change to json as it's entirely identical.

[1]: https://www.mediawiki.org/wiki/Requests_for_comment/Ditch_crappy_API_formats