[2021] The first assets and mod ever created

I don't know about the first assets, but the very first mod was All Spaces Unlockable
https://steamcommunity.com/sharedfiles/filedetails/?id=403798635

It's a lot harder to find as we have more assets on the workshop than the page counter can handle. But you can go into each category, sort by Most Recent, go to the very last page and see what the first one for that category was. Then compare the time and dates posted of the assets.

This was the very first prop:

https://steamcommunity.com/sharedfiles/filedetails/?id=410301792