Once we've done the first step of that (T102318) we can also research what the difference is between our current browser support and which ones don't fully support ES5. When we've identified those browsers and dropped support for them we can enforce ES5 requirement via a simple feature test for ES5 strict mode (e.g. es5-skip).
I expect the timeline for this to span the next 2-3 months depending on workload and more important quarterly priorities.
⇨ Test results (breakdown by browser) at T128115#3066522