Every individual feature has a support graph on MDN.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

Different browsers will update at different times (or not at all). There are websites like caniuse and mdn that will show you what browsers are currently supporting. If you are working on an intranet site, you might know that everyone will be using X browser that is on version Y. Otherwise...

If people are sticking to an older browser, they will never have the functionality (this can somewhat be mitigated through transpiling using Babel, but this isn't foolproof).

It often takes years for a feature to be widely deployed enough that no tranpiling is required. There are still features from ES2015 that I think still require transpiling.

need a compiler/transpiler that translates to browsers that don't understand these new features. and hope browsers catchup , which could be never.

So this is basically a fantasy what javascript could be no different than any other alt-js languages .