UE1 was in the timeframe that 3D acceleration was only starting to get adopted, and IIRC from some interview Epic continued with a software option for UT2003/2004 (licensed pixomatic?) because they found out a lot of players were still playing their games on systems where full GPUs weren't always available, such as laptops.

I know this is going back to Intel's Larrabee where they tried it, but I'd be real interested to see what the limits of a software renderer is now considering the comparative strength of modern processors and amount of multiprocessing. While I know there's DXVK or projects like dgVoodoo2 which can be an option with sometimes better backwards compatibility, just software would seem like a stable reference target than the gradually shifting landscape of GPUs/drivers/APIs