We can have a mission editor, but Unity doesn't really allow players to change models.
While mod support is about a lot more than just changing the models, the point of the question is that we'd need you to give us some way to tell the game to change things. In other words, a way to hook into what models are being used and tell the game engine to use a different one instead. That's completely outside anything to do with Unity.
Generally this means providing some kind of API that would allow us to tell the game engine to change which models/sprites/whatever to load, and add new ones to the game. Ideally there'd be ways to hook our own functions into the games logic and rendering loops so that we can control whatever we've added, or at least into some kind of event system that would let us know when different things happen in the game or when the user does something.
This definitely takes some effort, but has proven to be a huge boost to many games replayability and sales.
I agree that mod support would be great, but if it requires an entire separate steam workshop API to be built from scratch, that might detract a good chunk of the budget that might be better used to making the game the best, most polished it could be, with features the community are asking for that might exist outside of the original plans.
That being said, games of this sort always end up having a large and thriving modding community, and lots of people only even play the games because they can mod it, or because they want to mod. And games with a large modding community tend to maintain longer levels of interest than they might otherwise (see: every game over 5 years old with giant modding communities ever, as well as some of the newer games, like SDV and others.)
However, ultimately, tbh, I don't know anything about video game programming, only some web code, so I have no idea how difficult or involved making this sort of interface you're suggesting would be or how much budget it would require.