It's often legally feasible but the result is difficult to get value out of (and obviously worthless for the company).

For instance Frictional open-sourced their HPL engine, Penumbra: Overture, Amnesia: The Dark Descent and Amnesia: A Machine For Pigs.

However the Amnesia games are built with Autodesk's FBX SDK, and I don't think they include any assets, they're not necessarily uninteresting artefacts but there's not much you can do with them either.