Actually, I already provide this in the release notes. A PROTOCOL_VERSION bump means restarting tmux. We didn't do this between 1.9a -> 2.0, hence why it wasn't mentioned. Look at the release notes for 1.9, you'll see it was mentioned there. Simples.
Some of the information is out-dated yes, you'd expect that. Especially when people were asking for help about dial-up modems, for example.
But I bet if you were to try some of the other stuff on there, it would still work today...