There isn't a REST API command to shutdown volumio.
However it turns out IOS shortcuts support sending commands over SSH.
See the screen shots below. Create a new shortcut and in the search bar type "ssh" Select "Run Script Over SSH" Select Show More and you are presented with a form to complete - that will contain your variables Host is the IP address of your volumio User is typically volumio unless you have called yours something else Password is typically volumio unless you have changed yours Input is "sudo shutdown -h"
Click on next and give your shortcut a name - I called mine "Power Off Radio"
To shutdown the Raspberry PI on which I run Volumio I simply say "hey Siri Power Off Radio" The notification's app sends "sudo shutdown -h" to the pi and the pi powers off.