Queries without 'rvslots' get this message in response: "Because \"rvslots\" was not specified, a legacy format has been used for the output. This format is deprecated, and in the future the new format will always be used."
But online API documentation for the 'query' action doesn't have 'rvslots': https://en.wiktionary.org/w/api.php?action=help&modules=query
Did you forget to document it?
You're looking in the wrong place. The warning comes from the revisions module, and the parameter is documented there: https://en.wiktionary.org/w/api.php?action=help&modules=query+revisions
Some of the text on https://www.mediawiki.org/wiki/API:Revisions probably could use updating. That's a wiki, anyone should feel free to do so.
I see that some examples still need to be updated.
I've updated the main example in the MediaWiki's wiki documentation, but someone should also update the Examples section of Special:apihelp/query+revisions, defined as follow:
protected function getExamplesMessages() {
return [
'action=query&prop=revisions&titles=API|Main%20Page&' .
'rvprop=timestamp|user|comment|content'
=> 'apihelp-query+revisions-example-content',
'action=query&prop=revisions&titles=Main%20Page&rvlimit=5&' .
'rvprop=timestamp|user|comment'
=> 'apihelp-query+revisions-example-last5',
'action=query&prop=revisions&titles=Main%20Page&rvlimit=5&' .
'rvprop=timestamp|user|comment&rvdir=newer'
=> 'apihelp-query+revisions-example-first5',
'action=query&prop=revisions&titles=Main%20Page&rvlimit=5&' .
'rvprop=timestamp|user|comment&rvdir=newer&rvstart=2006-05-01T00:00:00Z'
=> 'apihelp-query+revisions-example-first5-after',
'action=query&prop=revisions&titles=Main%20Page&rvlimit=5&' .
'rvprop=timestamp|user|comment&rvexcludeuser=127.0.0.1'
=> 'apihelp-query+revisions-example-first5-not-localhost',
'action=query&prop=revisions&titles=Main%20Page&rvlimit=5&' .
'rvprop=timestamp|user|comment&rvuser=MediaWiki%20default'
=> 'apihelp-query+revisions-example-first5-user',
];
}