Wish this could be reduced lower. If I don’t unlock my phone in a day, something is up and extra paranoia is warranted.
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Edit: Actually, I was looking in the wrong place. It’s an option for the "Shut Down” action. Thanks, @jwond!
However, you can set it to shutdown or reboot if it receives an email that contains a specific string in the subject line. So you could have a computer that sends it a shutdown email. You could base it off of inactivity of that computer, or you could have it listen for a cancel-shutdown email, which could be sent by the iPhone when it connects to WiFi (assuming you don't live someplace like a university campus where you're always connected to WiFi even when you leave home.) If the computer doesn't receive the cancel-shutdown email within the set period of time, the computer would send the shutdown/reboot email to the iPhone.
However, like another commenter pointed out [1], the action doesn't even seem to work currently as of iOS 18.0.1. If you set it to "Run Immediately" with "Notify When Run" to off, it just doesn't run. If you turn "Notify When Run" on, the action runs, but still requires you to confirm it manually with a tap, which defeats the purpose of the automation. Who knows, maybe a future update will fix it.