> (EDIT: to my knowledge) the first major language with this approach was ECMAScript 2009 (5th ed.) with `use strict` and Python 3 predated it.

https://docs.python.org/3/library/__future__.html

That only works with “syntactic” updates, not with deep semantics changes.

Those are why the core team pulled the trigger on an overhaul update. And they were by far the biggest issues during migration, anything which would be gated behind an edition or opt-in was easy.