Well Unreal Tournament runs on Linux so it must use OpenGL. Or at least have support for it.
It was my understanding that the Quake and Unreal engines are the two 3D game engines most often licensed by other game developers for use in FPS games.
Yeah, the UT line is another one that supported both Direct3D and OpenGL (I'm sensing a trend here. Are ID engine based games and a few other select titles the only ones that are OpenGL exclusive?).
Note that UT 2004's readme says the following:
The OpenGL renderer is not officially supported on Windows but could be
a good choice on certain hardware / driver combinations as it might
trigger fewer bugs in drivers. Unless you are experiencing serious visual
flaws there is no reason to change to the OpenGL renderer though. Unlike
Unreal Tournament, Unreal Tournament 2004 was designed around D3D and
offers the best performance and visual quality with the D3D renderer.
Also please keep in mind that the OpenGL renderer has higher system
requirements than the D3D renderer. The OpenGL renderer is known to
not work correctly with pre-7.76 ATI drivers.
It seems to me that OpenGL use these days is pretty much based only on the developer's (or publisher's) desire to port to Mac or Linux or the use of one of ID's 3D engines (and even Valve modified their first engine to support D3D) and that few developers
are willing to go the OpenGL only route.