From Help Domino Designer:

JavaScript language elements (JavaScript) The JavaScript language elements are based on the ECMAScript Language Specification Standard ECMA-262 (see http://www.ecma-international.org/publications/standards/Ecma-262.htm). The JavaScript described here applies to the server-side interpreter. Client-side scripts are interpreted by the browser.

But not information about the year.

In which version of Domino can I use the "class" keyword, for ssjs library like in ECMA-262, 6th edition, June 2015?

I use Domino Designer V11. Maybe need some additional Designer settings for using class keywords?