It's the same story for Doom 3 engine / id-tech4. The most active fork I think is still Dhewm3, which includes some modern useful changes for compiling.
Well, Doom and Quake 1 are kind of in the sweet spot where they are still small enough that one person has some hope of understanding the code base. As impressive as the later IdTech iterations are, and as awesome as it is that the code was released, they are daunting for a hobbyist.