Currently most/all mods are external .dll files that are inserted into the game through an external third party mod loader named BepInEx. Unfortunately, the Steam Workshop does not really allow for certain kinds of external file types to be included, and as far as I know, .dll is one of them due to the huuuuge risk of abuse. (.dll's can ruin computers pretty easily.)

On top of that, the developers have no intention currently of implementing Official mod support- All mods are used at the players own risk. Given this and the above, it's pretty much a guarantee that Steam Workshop support will never arrive for this particular game.

If you want mods, Gotta get them from external sources like Nexus or Thunderstore.