Looked nice, but crossed it off as soon as I saw that, as I'm working on a project currently that uses many screens. Can't just call a thing legacy because you and the people you directly know aren't using it.
Nowadays, multiple monitors present one big virtual framebuffer and only one logical X11 screen.
The normal, usable way to have multiple monitors for your X11 desktop environment is for them to all be combined into one logical screen that you can move windows around in, and that applications aware of the right extensions can discover the actual physical layout of the monitors that comprise the single logical screen. Multiple screens in that X11 sense is a far more obscure feature than simply supporting more than one physical monitor.
What they are talking about is supporting more than one of those, and from app's perspective they are completely separate (can't move windows between them).
While I can see the use cases (say secondary screen only running single app) I never actually used that feature so it's understandable drop.
edit: AND i've been using GNU/Linux and derivatives for the last 20 years.