With formatversion=1, boolean response fields are typically returned as the empty string when true and are absent from the response when false. With formatversion=2, boolean response fields in JSON and PHP formats are supposed to be returned as native boolean true when true, and either native boolean false or absent from the response when false.
Any boolean response fields that use the formatversion=1 semantics with formatversion=2 should be fixed. Please file tasks in Phabricator if you encounter such fields.[1] Clients using formatversion=2 should be prepared for such response fields to be fixed without further warning.
[1]: https://phabricator.wikimedia.org/maniphest/task/edit/form/1/?tag=MediaWiki-...