ES6 (later rebranded as ES2015) simply was the first release after the language design process was ramped up. After vendors agreed to improve JavaScript and implement new features in their engines (which were possible before only using transpilers like CoffeeScript), lots of feature proposals were made and discussed under the term ES-harmony, with ES6 - the next revision of the standard that was expected to contain all those new features - being the big buzzword.
When ES6 finally came, it was a major release that included a plethora of new features, but not all of the previously discussed ones. The standards committee kept up their work, and changed to a new yearly release cycle (ES7=ES2016, ES8=ES2017, ES9=ES2018, …). Lots of fancy things were expected to be included in ES7 (the new buzzword, along with ES6+), but ultimately failed to be finished on time (see Is ES7 only composed of two features?). The TC39 committee then codified a process for adopting proposals which is still followed, and things get included in the specification releases (referred to as ES-next) when they're done. The importance of the standard revisions declined, features are denoted by the name of the proposal and not by the year they are expected to get released.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…