It has been possible on Linux for way longer than that, Xnest dates back to when it was still XFree86 rather than Xorg. Quick googling has Xnest and Xephyr mentioned in 2012 changelogs (X11R7.7), but those aren't even "creation" changelogs. (I can't easily dig up when they were created…)
eqvinox|1 year ago