As of 2015-05-04 it says

Many modules have changed result data formats. While this shouldn't affect clients not using the experimental formatversion=2015, code using ApiResult::getResultData() and not using ApiResult::transformForBC() may need updating.

but it's formatversion=2 and there's no transformForBC() instead there are META_TYPEs BCarray, BCkvp, and META_BC_BOOLS. Also e.g. extensions/EventLogging/includes/ApiJsonSchema.php doesn't use getResultData() (it calls getResult() and $result->addValue() ), yet it needed updating with META_BC_BOOLS to fix breakage.

@Anomie cautions this should still be flagged experimental in 1.25 since there may be extra changes if results get improved.

https://www.mediawiki.org/wiki/MediaWiki_1.25#API_changes will also need work.