It doesn’t necessarily have to be open world as is currently used these days. The OG Doom isn’t exactly linear, but also isn’t open world in any sense. Remove the loading times between levels and it would be open world in the way that term was originally used.
viking@infosec.pub 41 minutes ago
The OG Doom is fairly linear, unless you play on the lowest difficulty level where all doors are permanently open. Else you need to kill specific enemies that can only be found in certain rooms to get keys.