I don't have a whole lot of information, but I can tell you what I know.
I've had a developer key to S&Box for quite a while now and I can say the game is not finished. It still has a lot of things that need to be done, rather they be large updates or normal things you should see in a game. I haven't been on S&Box for a little bit now but when I was playing, water did not have any sort of physics, a default prop list has not been creating and custom content has not been added.
When I refer to custom content I mainly refer to the fact that the default guns in the game utilize the same models as the guns you would find in rust. I believe this game will be finished sometime mid 2022.