[hnbad]

`indexes[haystack.indexOf(needle)] = true`

There's a valid example of code that would be broken (`indexOf` returns `-1` as "not found"). Is it a good way of solving whatever the author was trying to do? Probably not, especially now that sets exist. Is it code you might conceivably find on hunreds of sites across the past decades of the world wide web? You bet.

Yes, we could introduce another "use strict". But we only just got rid of the one via ESM (which enforces strict mode). That was a one-off hacky solution to a hard problem coming off the end of a failed major version release of the language (look up ECMAScript 4 if you get a chance). We don't want to see a repeat of that.