I honestly have no idea what you're talking about, I still see an option for "Allow different input sources for each window" in the keyboard settings panel.
I'm posting that because (a) I believe that is the issue referred to, (b) if you look at the reactions there is definitely a lot of "why would you even want this", there is the "lower cognitive load", and they did go ahead with a release in which it was broken, even though it was reported before the release was finalized.