Why it's 2.0? I don't see any really significant changes, or maybe I missed smth.
Incompatible Changes
====================
* The choose-list command has been removed.
* 'terminal-overrides' is now a server option, not a session option.
* 'message-limit' is now a server option, not a session option.
* 'monitor-content' option has been removed.
* 'pane_start_path' option has been removed.
* The "info" mechanism which used to (for some commands) provide feedback
has been removed, and like other commands, they now produce nothing on
success.While trying to use the same .tmux.conf over multiple versions of tmux 1.x, I often had to adapt the config file to settings being renamed or just going away without a deprecation warning beforehand.