Wikidata has a help page for almost a year now : https://www.wikidata.org/wiki/Help:Monolingual_text_languages where it's clearly stated new code are ok, as long at it's a valid BCP 47 code and used in a real case. And it's written : « A language code does not have to fulfill the requirements of the language proposal policy for new Wikis. » ; and indeed, a lot of lang code where added, AFAIK the lang com never even talked about it (at least not when the « und », « mis », « mul » and « zxx » codes where added last year : https://phabricator.wikimedia.org/T72205 ).