Is there a modern browser for iOS 10.3.3 capable of supporting ES6+ Javascript?
From what little I know about ECMAScript Safari on iOS (which all browsers must use for rendering web pages, as @Mark pointed out in his comment) some things are supported but many are not. And your guess is as good as anyone's as to why that is the case (not to mention off-topic here).

So you are likely out of luck for full ECMAScrip support on iOS unless you want to go the jailbreak route. We can't help with that and I don't know if there is a browser for jailbroken iOS devices that does support ECMAScript.