That's why I break out things that are platform-dependent. E.g.:

   if-shell "[ $(uname) = 'Darwin' -a -f ~/.tmux.macos.conf ]" "source-file ~/.tmux.macos.conf"

Technically `reattach-to-user-namespace` is a macOS-specific hack to get tmux working, so there's no need to jam it into your standard config.