OS を sierra にしたら、tmux.conf の以下の設定で warrning が出た。

set-option -g default-command "reattach-to-user-namespace -l bash"

ので、brew upgrade で対応。

$ brew upgrade reattach-to-user-namespace
==> Upgrading 1 outdated package, with result:
reattach-to-user-namespace 2.5
==> Upgrading reattach-to-user-namespace
==> Downloading https://homebrew.bintray.com/bottles/reattach-to-user-namespace-2.5.sierra.bottle.tar.gz
######################################################################## 100.0%
==> Pouring reattach-to-user-namespace-2.5.sierra.bottle.tar.gz
🍺  /usr/local/Cellar/reattach-to-user-namespace/2.5: 6 files, 34.1K
$