When you next --sysupgrade, you'll get a "file conflict" warning about the su binary and its corresponding manpage. This is because Arch now uses the su binary from the coreutils package instead of the one from the shadow package, as it is more functional.
You can safely force this upgrade by running pacman -Suf. When the upgrade is complete, make sure you still have a /bin/su binary before exiting your root shell. Just in case. ;)