[Garlef]

> Why is there still no simple way of handling changes like this?

This is nothing JS specific. Breaking changes are breaking changes. If you can, don't introduce them.

> simple way to have a header in each file with the language version

One special aspect that differentiates JS from other languages:

It's both a language AND a universal runtime. A lot of JS that's executed is not JS that's written by humans but generated by a compiler/transpiler.

So adding a layer of header versioning is not a big win in terms of developer experience: It would anyways be the deployment toolchain that's responsible to deal with such a versioning scheme. It would ideally be invisible to the developer.