From what i see there are no mods outside Steam because you have no working forum (or at least the one that your site links to can't be opened) and the game itself actually makes modders upload to Steam workshop. Unfortunately when i try to download one of them (via Steam API or using service like steamworkshopdownloader) i get the following error:
The game that this item belongs too does not allow downloading of its items...
Can you please correct this. After all you _DO_ sell the game via other distributors. Actually i would have never brought it in first place if it was Steam only.
It's not Steam only, there's nothing steam specific about the modding system at all. It does support Steam workshop, but you can zip up a mod and install it in the save path in the \Mods folder without Steam involved. Of course you can't access Steam-specific features if you don't have a Steam version.
There's an active discord, with an active modding community: https://discordapp.com/invite/cavesofqud