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There will always be a low-level reboot/shut off mechanism which bypasses the iOS software, so no matter you implement FaceID or passcode to shutdown, smart thieves can always find a way.
 
No. They can just let the phone battery die by itself if they wanted to. But with the latest update, it's still trackable.

Here's an article about it:

Most thieves wouldn't care anyway. They will want to get rid of the phone fast, or at least for parts, to get cash quickly. So they will just sell it off somewhere, regardless whether the phone is on or off.

All that matters is as long as they didn't guess your passcode, your data is safe. Simply enable lost mode from icloud.com.
 
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