Thank goodness it's not a feature that can't be removed. We were very distressed by not being able to span an application window across screens. And one of us was not happy about the menu showing up on top of both screens.
I am curious about this now. I might like the new feature. But for my friend... taking it back to old-style is what she wants.
In System Settings, choose Mission Control. Default is "Displays Have Separate Spaces" checked. Uncheck it and re-log-in or reboot. You will be able to span windows across multiple screens again.