Not true:

http://www.cs.cmu.edu/~412/lectures/2009-10-23_radeon.pdf

>>Provide 2D acceleration via the 3D engine.

I stand corrected, however 2009 is already past the time I was paying attention to Plan 9.

As explained in the abstract of the paper I linked above, 8½, the Plan 9 window system, has supported 2-D acceleration since at least 01992. I also mentioned this in my comment, which additionally outlines how this acceleration support works across networks.