It looks like mainly a Hyprland fix (and maybe some wlroots based compositors). The old method still works with sway for me, and there's a another approach using a virtual edid that should work everywhere, but perhaps not with Nvidia cards (see here: https://discuss.kde.org/t/how-to-create-a-virtual-monitor-display/2725/5).
I'm not sure if Plasma or Gnome have any support for headless monitors outside of the EDID method.
Yes, although the approach that was fixed only applies to Hyprland and some other wlroots compositors. You can use the virtual edid approach on other systems, but it may not be supported on Nvidia GPUs. You can also use it as a simple supersampling method, such as rendering at 1600p to a Steam Deck, for example.