It's added as an optional feature, as the first sentence of that answer describes: > ...the answer below is no longer accurate. _proto_ is being added to ecmas

It's added as an optional feature, as the first sentence of that answer describes: > ...the answer below is no longer accurate. _proto_ is being added to ecmascript6 as "normative optional" which means it isn't required to be implemented... It can't always be polyfilled, either, which makes the feature effectively useless. I'm not even sure why they added it as such a half-hearted feature to ES6; I suspect politics (Brendon Eich appears to have been dead set against having mutable protos for years).