Most of the mod community is currently contained on Steam, which isn't accessible to GOG customers, or possibly other platforms in the future, as easily. So there's currently very little reason to buy the game outside of Steam, even when the game is discounted lower, which feels like non-steam owners kind of get the worse deal in the end.

I checked a few older posts on this topic, and the original response was that Steam Workshop was easier for consumers, but now there's other options that are easily available to all consumers across platforms.

With the rise in cross-platform mod support via Mod.io, would this ever be implemented into the game?

You can download mods (I think most of them) with SteamCMD even without steam account.

Yeah, but that's still quite an annoying process to go through, since you also need to download each workshop item individually with a command and then you need to worry about updating them all manually, to be able to join multiplayer modded servers.