As an aside, is that really a problem these days? All modern browsers now offer comprehensive native ES6 support. Unless you still need to support older platforms like IE or some of the legacy mobile browsers, it's mostly a non-issue.
Like you said, it depends how much you're betting on your JS to work perfectly, and who your intended market is. If all you're doing is hoping for the most optimistic JS runtime (and perfect connectivity, high bandwidth, etc.) and your site is a white screen of death otherwise, then you're not really building a robust piece of software for the web. Of course, that may not be what you're trying to do, then it's a non-issue.
Presumably IE11 will hold out forever, since it's not under meaningful development any more.
What's missing from any recent version of iOS Safari, though? Anything from 10 upwards supports pretty much all of ES6 with very minor exceptions, AFAIK. Even features from more recent versions of ES tend to be supported quite well quite quickly in the era of self-updating "evergreen" browsers.
Unfortunately it uses ES6 features. There's no ES3/ES5 demo without JSX -- https://reactjs.org/docs/react-without-es6.html uses JSX