You don’t need to learn ES6
ECMAScript 6, also known as ECMAScript 2015, is the latest version of the ECMAScript standard. ES6 is a significant update to the language, and the first update to the language since ES5 was standardized in 2009. - Luke Hoban
So… do you need to learn ES6? No. You don’t.
Especially for application development ES6 has a ton of awesome new features. But you can build apps 100% vanilla with ES5. For DOM manipulation, ES5 is also more than sufficient.
Backwards compatibility still sucks, so if you do use it, you need to run your code through a processor like Babel to get JS that can be used by more than just the latest browsers.
Now, this doesn’t mean you shouldn’t learn ES6. Understanding JavaScript more deeply is a great thing.
But not knowing it won’t hold you back from writing awesome scripts and plugins.