The project has gone through much since it was last updated, and the old code base (which wasn't much of a base) had to make room for the new.
The multiplayer patch isn't the end of single player content, I've just moved the old branch to a beta that you can opt into.
Right click the game in your library, then go to Properties - > Betas, click the drop down and select "legacy_content".
I'm working on single player content for the new networked codebase, I'll hopefully have some stuff to share in the next week or two.
Thanks for reading, the new year is almost here and I wish you all a creative one.