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.
But not knowing it won’t hold you back from writing awesome scripts and plugins.