The Xorg state tracker originally had a goal in the Gallium3D world of providing 2D acceleration support via Gallium3D drivers and to serve as a generic mode-setting driver for the X.Org Server. While the concept was great at the time, the Xorg state tracker hasn't seen wide adoption and there's other software components out there that do the job.
The Xorg state tracker was used by VMware for their virtual graphics driver, but now they use the XA state tracker in its place. The Freedreno project is also looking at using XA for some 2D acceleration support too.
Coming about independent of Mesa/Gallium3D drivers but optionally supported by the Intel and Radeon X.Org drivers -- and now mandated by the Radeon HD 7000 series GPUs and newer for 2D support -- is the GLAMOR library. GLAMOR also achieves 2D hardware acceleration via 3D/OpenGL.
For generic X.Org (DDX) drivers, there's now xf86-video-modesetting and other options.
Long story short, the Xorg state tracker hasn't been widely used by Gallium3D drivers, the code is suffering from bit-rot, and there's other options out there to replace the purposes served by this early Gallium3D state tracker. As a result, as of tonight the Xorg state tracker is now in open-source graphics driver heaven. The code nuking happened with this Git commit and now Mesa is seven thousand lines of code lighter.