The Doom engine uses a lot of global mutable state. In context, there's nothing wrong with this. Why pass around state explicitly when there's just one instance of the state? Multithreading wasn't on the horizon for game engines in 1993. Single CPU cycles were very much a concern. Developer time was also limited (most of this code was written by a single person in about a year). This wasn't a toy project that could have evolved in a million different ways, and it wasn't a crypto library either. There was a game that had to be shipped, and John Carmack was surely more concerned about getting the game engine to do precisely what it needed to do to run the game than proofing the code for hypothetical future scenarios.