Ultimately, it depends on who you're making the app for. All major browsers support ES6 at this point except Internet Explorer. Microsoft is removing support for IE in a year, but as far as we know, people will still be able to use it after that if they want to. There are a significant number of people in the world still using IE because it's what they know, what they're used to, and learning something new is intimidating. The question is: do you want to make your app accessible to that group of users?