if you use your old rust files, you might be able to still join it. We did that last week. our server hasent update for 7 days because the previous update ( july 9) didnt triggered the autoupdate from servers.
We simply copy pasted the Rust folder from our friend who didnt update. and then launched Rust.exe directly.
I know there is a way to still run the old version and run it through steam, i managed to did it yesterday, but its hard to reproduce. its hard to launch an old version of rust through steam without steam to force you to update the game. buts its doable.