Polishing for Plasma 6.6 (due in 10 days) continues, that the Window List widget gets sorting and section headers, System Setting loses all inapplicable hardware pages, and the window manager remembers tiling padding per screen, among many other marvelous things.
I suspect there’s something big going on that won’t make it as easy as you’re thinking. I’d bet it’s the same reason they haven’t been able to bring one-wallpaper-per-virtual-desktop back since KDE4. They focused on Activities and worked really hard on that trying to make it a thing but it seems most people didn’t really cared about it, and almost forgot about virtual workspaces in the process
KDE4, funnily enough, implemented it as “entirely separate set of widgets per virtual desktop”. Which would also be nifty, but I can understand why it might be hard to do.
(The desktop wallpaper? That’s a widget, of a sort. It goes in ~/.config/plasma-org.kde.appletsrc along with your panels and widgets.)
– Frost