Comment by cogman10

The decided not to have it there because they didn't like the idea of maintaining version 4.0 forever in their engines.

That's basically why they never did anything like "use strict" again.

IMO, that's a bad choice. Giving yourself the ability to have new behavior and features based on a version is pretty natural and how most programming languages evolve. Having perpetual backwards and fowards compatibility at all times is both hard to maintain and makes it really hard to fix old mistakes.

The only other reason they might have chosen this route is because it's pretty hard to integrate the notion of compatibility levels into minifiers.