Strict mode was opt in though, and code would still evaluate without it.

There's also a lot of new syntax though, which does have high costs in cases like generators, and shipping the regenerator runtime because you need your code to run everywhere.

To my knowledge they have extended syntax and added modes, but not actually made any breaking changes.

A mode was a way to avoid making breaking changes.

What I missed from my comment is not just about the engines in use, but also that all old spec confirming JavaScript should still run, in all new environments.

That is backwards compatibility.