Earlier quoted context omitted.
What makes you think this is unlikely? On the contrary, I think it's _very_ likely there are things depending on it. I don't expect there to be a huge number of them, but I also expect them to disproportionately be in things like intranet deployments where it's hard to even get measurements. :(
A link with _blank + expected to change/interact with opener? Maybe there's a few, but most cross domain transports would use window.open() manually.
We could have different default behaviors for the two cases, of course: opt-in for links and opt-out for window.open.
But I still suspect that even just the link case is not as uncommon as one might wish. Happy to see data proving me wrong, though!