So I see there is the Hd Mod Category. Recently I've been playing the GoG Version of Diablo. Is that what the HD Mod refers to? Because there is a mod called belzebub but it's also referred to as "hd mod" so I'm not sure.

Heyo. GoG version is still the standard version of Diablo. Regarding HD Mod, I think it's belzebub mod. There is a Tchernobog mod as well and frankly I'm not sure which one is "the HD Mod" listed in these categories.

Yes, current submitted run in "hd mod" it's a Belzebub mod. It should be placed to "modifications" page https://www.speedrun.com/diablo/games . I work on that.

Also there are some other mods that can be placed there (like Tchernobog) but need to find runners first.

We have at least a couple of DevilutionX runners. How many should there be before it gets moved?

Belzebub runs were moved to separate mod page. Soon it should be indicated as "unofficial" game and added a tag "modification". https://www.speedrun.com/diablo_1_belzebub_mod