Maybe we should re utilize the "use strict"; type of metadata. Something like "use es2021" to enable negative indexes etc.
Unfortunately, I couldn't find any links to articles written at the time about this, but they definitely did consider "use" options or script type="es2021" kinds of options.
0. It will end up completely unnoticed, because everyone is using any language you like and compiles to web assembly, which is the defacto standard for more than a decade.
1. It's like a new C++, because of the many TC39 stage 3 proposals that has been added over the years without the possibility to ever fix the language in order to not fork the web.
Imagine learning JS in 5 years: "you can index arrays with subscripts, but actually, if you want to get negative indexes you need to use .at()"
vs.
"Always add 'use es2021'; at the top of your scripts. You can index arrays with subscripts."
I'm hardly a huge Perl fan, but their approach made a lot more sense IMHO and is a good trade-off between making sure existing code works, and not complicating the language for future use. The "fork" (which seems a bit hyperbolic to me) is a very minor short-term pain at best for significant long-term gains.
I don't know of any other mainstream language that's so conservative as JavaScript in never breaking anything, even optionally with flags.
Besides, compatibility is important but not holy. Some programs probably also rely on "[9] * 2" resulting in Number 18, but we really ought to fix that (with or without flag) IMHO because these sort of gotchas result in people writing bugs every single day where it works for an array length of 1 and then it has 2 values and you get NaN. The pain of breaking backwards compatibility is minor compared to the pain of new bugs being created every single day.
Golden opportunities are being missed here, and it's a shame.
I absolutely agree that some metadata at the top of a module enabling behavior like this would be ideal.
From a spec perspective, ES6 modules were a good milestone to "flip the switch" over to strict mode by default, but even with that being a fairly successful strategy (IMHO), it still left some nasty corner cases around the language (namely, there are now two distinct top level grammars, which led to the whole .mjs bikeshedding rabbit hole)
We need more special contextual comments to change a file or scope to behave better so we can move forward and scrape away all the bad legacy of JS.
I don’t think making the JS world into even more of a “set of subtly different languages that look mostly similar” is necessarily a solution so much as an extra problem.