It seems that byobu triggers a known bug with tpm which then fails to load any plugins. Fortunately there is a simple fix, in byobu’s .tmux.conf change your plugin formatting like so:
set -g @tpm_plugins ' \
If using the XDG directory convention, this line is also needed in .tmux.conf:
set-environment -g TMUX_PLUGIN_MANAGER_PATH "~/.config/byobu/plugins/"
So it looks like MacOS Sierra broke pbcopy/pbpaste for tmux. This manifests as clipboard errors when using netrw/vinegar in neovim under tmux. (Dunno why netrw is writing to the clipboard but anyways).
The solution is to use Homebrew to reinstall a patched version of reattach-to-user-namespace:
brew uninstall reattach-to-user-namespace
brew install reattach-to-user-namespace --with-wrap-pbcopy-and-pbpaste
Although this was annoying, it did lead to running brew cleanup which reclaimed 3.8GB of disk space. Jinkies!