The X Window System (X11) is a protocol with multiple implementations. Sure, the X.Org Server (Xorg) was the most popular by a huge margin, but there were quite a few others (e.g. XFree86, Xming, XWayland), though over time most were discontinued for one reason or another.
X11 and Wayland do differ in an important way: in X11 window managers (GNOME, KDE, i3, whatever) all sat atop the Xorg server; whereas in Wayland there’s only the compositor, so GNOME, KDE, Sway, whatever, all essentially include their own equivalent of Xorg (which could be fully integrated, or factored into a library, such as Mutter, KWin, wlroots).
There were plenty of those, including commercial ones.
It's pretty hard to find but ~25 years ago I was using Xi Graphics Accelerated-X which had 3D acceleration long before Xfree86.
Update: but yes I imagine it had some code from original MIT release.
For completely independent one you can have a look at WeirdX/WiredX, which was written in Java and even supported antialiasing and transparency for core protocol (something that Xfree86 people claimed to be impossible to implement).
The X Window System (X11) is a protocol with multiple implementations. Sure, the X.Org Server (Xorg) was the most popular by a huge margin, but there were quite a few others (e.g. XFree86, Xming, XWayland), though over time most were discontinued for one reason or another.
X11 and Wayland do differ in an important way: in X11 window managers (GNOME, KDE, i3, whatever) all sat atop the Xorg server; whereas in Wayland there’s only the compositor, so GNOME, KDE, Sway, whatever, all essentially include their own equivalent of Xorg (which could be fully integrated, or factored into a library, such as Mutter, KWin, wlroots).