The 'headitems' property for action=parse has been deprecated, and using it will return a warning. If constructing a page from scratch, you should use prop=headhtml instead. If updating an existing on-wiki page, use prop=modules|jsconfigvars and load them into the existing ResourceLoader code with mw.config.set() and mw.loader.using().

Also, the 'headitems' property will no longer return code related to ResourceLoader. Loading ResourceLoader would require additional data that is not sent by 'headitems', and given the deprecation it's not worth the trouble to add that data.