X11 cannot handle the average user connecting an average laptop to an average external monitor (no usable per-screen scaling). Maybe this was good enough in the 80s, but today it is seriously holding back the platform.
But even with that it's STILL possible with the right setup using xrandr where you essentially render at a higher res and then downscale. Ubuntu's X11 version of Gnome has had this out of the box since I think 20.04 and it works very well. IIRC upstream Gnome refused it because that's what Wayland is supposed to do...
Yes and no.
X11 screens could have different resolutions, color modes and pixel density, but windows could not span over multiple screens, or moved from one screen to another. The only way the user/the application could move window to a different screen would be to open a new connection to the screen (denoted by that familiar DISPLAY:0.x environment variable) and recreate all the resources there.
There was exactly one application that was capable of doing that at runtime (XEmacs). For all the others it meant restarting the application with a new DISPLAY env var.
Hence Xinerama. It joined all the different physical displays into a single screen, which allowed to move windows around, but came with limitations, like the same color modes or DPI for all displays -- since it was single screen logically.
You actually don't even have to do this for updated programs - hidpi aware applications scale themselves, vector graphics style, so there's no up then down scaling going on. And applications can easily read the xrandr config and adjust their factor when moved to a different monitor. However, xrandr's ppi factor is not the scale factor you likely want, so this isn't really standardized; each toolkit might do it a bit differently. But all the pieces are there.
Non-aware applications might be bitmap scaled if needed though. (Of course, wayland just breaks all legacy applications anyway so that sets the compatibility bar low regardless)
Screen sharing is the main gripe, it’s a pain to configure, involves a lot of different bits of software which have to be orchestrated and none of them are mature enough not to break occasionally in an unexpected fashion.