i think it might have something to do with the fact that when you write client scripts, you are relying in the browser JS engine. Is there any netsuite API only available on 2.1? I don't think so, right? it's mostly just adding newer syntactic sugar not related to netsuite capabilities. It might be that NS is trying to avoid giving you a false sense of security that using the latest ECMASCRIPT capabilities will work in all browsers, because they won't transpile/add polifills on client scripts for you.