Anecdotally I wrote some JavaScript just today (I'm not a frontend dev but I write some simple stuff for fun sometimes). It seemed to me that every modern feature you'd care about is now widely supported. I used the JS fetch API and template strings and these things just work in modern browsers. I even encoded my web fonts in WOFF2 only, because it's so widely supported that I didn't see the point in bothering with WOFF1 too.