Ion Fury uses a modified version of eDuke32 which can do software only rendering, and quite well, but defaults to GPU acceleration because Ion Fury uses the real time lighting.

That said, the Build Engine had software voxels in 1997. Which was pretty cool.

And for those who want more of this type of game I must absolutely recommend the game “Supplice” as well.

My understanding was they removed that software renderer specifically because it didn't run well.