The reason that Doom is so portable goes beyond Linux and is an artefact of its development. id developed Doom on NeXTSTEP (i.e. Unix) machines and obviously targeted DOS. This is pretty unique among DOS games at the time and required id to write as much code as possible in a platform agnostic way. This means that the main engine does not care about where it is running and the usual DOS hacks are contained to DOS-specific files. In order to port Doom to a new platform, ideally one only needs to rewrite the system-specific implementation files for video, sound, filesystem access, etc., and this mostly holds true today. (These files are prefixed with i_ in the Doom source).

The Linux port is just one of many versions developed at the time. I don’t believe that it was commercially released; it was more of a portability test. The reason that the Linux version was chosen for the source release over the DOS version was because it didn’t rely on the proprietary DMX sound library that the DOS port used.

Hexbear is an instance formed mostly by former r/chapotraphouse users after that sub was banned from Reddit a few years ago. Hexbear used to run on a custom fork of Lemmy so that the community could add extra features that they wanted (like custom emoji) but it was recently ported back to mainline Lemmy after merging or reimplementing as many changes as possible.

Currently, Hexbear does not have federation enabled, and there is discussion about who to federate with or even whether to federate at all. The community is very active and self-sufficient and some members prefer the isolation.

Content-wise, it’s a leftist-focused instance. Some shitposts, some serious posts, and a lot of inside jokes.