Is there any documented attack on macOS or Windows that utilized window title information?

Never in history. If you have software running on your system attacking you then you have so many more issues than the adversary knowing your window titles.

Sure, but again there's no interest in actually making a standard way to do it. I can understand it being something that arbitrary applications probably shouldn't be able to access, but that somehow turns into no way to do it, or complete fragmentation where every DE does it with arbitrary differences (or, more realistically, some DEs support it and others don't).

Okay; is there a way to do it? Can I, the user, decide that I do actually want a program to see titles? Or is it still impossible because 17 years isn't enough to implement utterly trivial APIs that people want?

You are aware that by default programs you run have access to your entire home directory, right? Applying any restrictions to them in the windowing protocol is security theater.