With the merge of Gerrit change 154098,[1] various seldom-used API output formats are now deprecated: dbg, dump, txt, wddx, and yaml. Clients using any of these formats should transition to the json format.
In addition, any clients still using deprecated parameters to other modules[2] should transition to the non-deprecated replacements. We've begun logging the use of these parameters on WMF wikis, and when usage drops sufficiently they may be removed.
Plans are also underway to improve the json format.[2] This may result in the deprecation of format=json in favor of format=json2; if you would like to provide input on the planned changes or on whether the improved format should be "format=json2" or something else, please comment at the linked page.