* wddx and dump - in 6 months (around May 12, 2015)

* yaml, txt and dbg - in 1 year (around November 12, 2015)

They are already deprecated which means that every request made with them receives a warning about deprecation. So on aforementioned dates, these formats will be gone from WMF sites and the removals will be part of next MediaWiki releases (presumably, 1.26 and 1.27). After that, only 3 formats will be left: json, xml and php. Ideally, we would really love to have only json, but currently there are no plans of doing it any time soon.

== Migration guide ==

The preferred format for using with MediaWiki API is JSON. The easiest way to migrate would be:

* "YAML" as used by the API has actually been JSON for years, so just replace format=yaml with format=json and make sure that your code does not barf at content-type set to application/json instead of application/yaml.

* WDDX is a type-aware format so the logical transition would be JSON.

* dump and txt were are "serialising" using the PHP functions that were never supposed to return anything machine-readable; and while format=dbg is kinda machine-readable, passing its output to PHP's exec() is not the way of machine-readability any security-aware person would want :) For any of these formats, just sticking with JSON would make your life much, much easier. However, if you're relying on specifics of exported PHP state in your PHP-based client, format=php might make sense for a quick switch.