has anyone started to use SuiteScript 2.1?

Yes, lots of people have.

Yes. It's my default choice

company I worked at refused to change from 1.0 and of course, I have a good base of 2.0 but what are the main differences from 2.0 to 2.1?

The main difference is the JS engine, and support for ES10 vs ES5.1

thank you, everyone!

To highjack the thread, is ES naming by year or by version number? or both?

ES2015 = ES6, ES2016 = ES7, ES2019 = ES10. Assuming the releases keep coming once a year, that pattern should continue. Prior to ES6. there were long gaps between ES versions, and some proposals, like ES4, were never adopted.

Yeah I understand the equivalencies, I'm just curious which one is "official", or maybe they both are

According to a few online sources (not sure if this is definitive), the official names are ECMAScript 2015 Language, ECMAScript 2019 Language, etc.

I would imagine the version number is official and the year just happens to be (even thought intentionally) moving at the same rate.

Ironically, in the early years, new ECMAScript versions were coming every year (1997, 1998, 1999), then a decade gap because of disagreements about ES4. Then another 6 year gap after ES5 in 2009 until ES2015. Things have sped back up again since then, which is great.

yeah looks really strange looking at it in hindsight

The ES4 debacle was due to different companies trying to push their own standards. Much like the browser wars, it stifled technology innovation and growth, because companies were trying to get people locked-in to their specific tech stack.

I think today, unfortunately, individual features progress independently through the standards process