It would be useful to have an option for kill-pane and kill-window to affect dead panes/windows only. For example: To bulk clean-up all dead panes and windows (those with remain-on-exit on) without affecting live panes, it would be quick and easy to use a -d option:

tmux kill-pane -ad
tmux kill-window -ad

Likewise, if -d is used with -t it would only kill the target pane/window if dead (but if -dt is used with a live target, the exit code should probably be non-zero).

It should also be clarified in the documentation that kill-pane only affects the current window, and kill-window only affects the current session.