Disable 3D in the editor - endlessm/threadbare GitHub Wiki
Since Threadbare doesn't use 3D at all, it is useful to hide the 3D tab from the editor to reduce confusion.
To do it, open the menu: "Editor → Manage Editor Features…":
Then create a profile (if you haven't done it before) and untick the "3D Editor" feature:
Now 3D is gone from the main editor tabs:
In this case I have also the Assetlib tab hidden, but you don't have to!