The new command is more portable.
# Nothing left of window list.
set-option -g status-left ""
# Current load average and hostname on the right.
# Nothing left of window list.
set-option -g status-left ""
# Current load average and hostname on the right.
-set-option -g status-right "#(uptime | cut -d: -f5- | tr -d ,) #H"
+set-option -g status-right "#(uptime | sed 's/^.*load averages*: //; s/,//g') #H"
# Format for windows in the window list in the status line. #I window index,
# #W window number, #F window flags.
# Format for windows in the window list in the status line. #I window index,
# #W window number, #F window flags.