Yeah, OP's example of Doom had the original sound engine library ripped out of the source code for release: https://github.com/id-Software/DOOM/blob/master/READ

Yeah, OP's example of Doom had the original sound engine library ripped out of the source code for release: https://github.com/id-Software/DOOM/blob/master/README.TXT#L12 https://github.com/id-Software/DOOM/blob/master/README.TXT#L... Similarly Doom 3's source code release had features ripped out: https://github.com/id-Software/DOOM-3-BFG/blob/master/README.txt#L42-L52 https://github.com/id-Software/DOOM-3-BFG/blob/master/README... And that's a 20 year old game which id made the engine for. A 10 year old game that used Unreal Engine 3 would need permissions for Unreal Engine 3 for the source code to not be useless.