Installing redshift, and running redshift-gtk, I got the message "/usr/bin/env: ‘python3’: No such file or directory." Unless there is a policy about not adding all recommendations for a package if they return a simple error when run, then python ought to be mentioned somewhere in the package (it didn't pull the other optional dependencies), and I didn't have python on this relatively new Arch system (despite it having XFCE, i3, vim, and Firefox).