It is also a breaking change to use new syntax and functions since old browser does not support new features. In this perspective `arr[-1]` seems a fair breaking change.
No, because changing browsers to interpret `arr[-1]` as `arr[arr.length - 1]` breaks existing sites that expect `arr[-1]` to be interpreted as `arr['-1']`: That is, the value stored on object `arr` at key name '-1'.
Changing browsers to interpret `arr.get(-1)` as `arr[arr.length - 1]` doesn't affect any old code using `arr[-1]`.
It's not about supporting old browsers. It's about supporting old code.
I think you're confusing your application with the language itself.
Adding new syntax and functions to the language is not a breaking change. Old code will continue to work.
If you start using these new features in your application, and it no longer works on old browsers, then sure that's a breaking change. But that's a choice for you to make. The language is still backwards compatible.
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.
