A couple of examples:
- Irrlicht 3d engine openly claims support for various Quake formats, among others
- Blender openly claims support for a multitude of model formats, including Quake's md2
- Assimp asset loading library openly claims support for Quake's models as well as others
I think the main issue is whether the file formats contain patented features in which case it would be illegal to implement in some countries. This is most frequently encountered in formats that attempt some kind of compression, such as audio/video files.