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I think even if I preferred a single screen I'd keep another one around so more easily screen share and be able to keep some things offscreen.

Virtual desktops can do this. Just share one virtual desktop and not the others.

Which operating system(s) or window managers are you thinking of?

I don't believe Gnome provides this option.

I would expect the ability to share only one virtual desktop would be a function of your display sharing software, not your desktop.