Environments do not run "a particular version of the standard." Rather, environments implement features from different years of the specification piecemeal. For example, it may be that in June 2022, a particular browser has implemented, say, 7 of 9 specification additions or tweaks from the prior year - and a different browser will implement those things on a different schedule.

Even if something is in the standard, there's no absolute guarantee that environments will support it, unfortunately. Usually they will, and often in a reasonably timely manner, but not always. For example, proper tail calls, from ES6, still have not been implemented in V8 or SpiderMonkey.

So, to figure out what is supported in a given environment, you'll have to test all the properties/features you're looking for individually. (eg window.hasOwnProperty('Promise'))

Often, rather than script-writers trying to detect what a browser currently supports, and working around that, script-writers will simply use a polyfill service (example) that adds all necessary missing features - that way you can get on with the main logic of your application and simply not worry about it.

Vanilla JS does not have any version as of Jquery,Angular, Reactjs, etc which are build using vanilla JS. It depends whether the current browser update supports upto which version of ECMA Script. You can use any version syntax as long as you can compile your code to ES5 syntax if your browser does not understand higher version of JS.