tldr; "Maintenance Mode hides all of your personal data, including your installed apps."

Hm... iPhones don't need to be unlocked in order to be serviced (although you need to remove Find My iPhone).

Also deletes any data added or created while in this mode, presumably to inhibit spyware

I would like a "handed it to my parents to look at the baby photos" mode.

In before it’s on like three of their 20 models.

How about a -trade in mode- so when my package arrives at Verizon “empty” we can both locate it.

This could be possible...

1. In Control Center, put an icon of a semi-lock.
2. When enabled, the app you're in remains available, but if you exit the app, the phone locks.
3. Additional restrictions may be defined such as inability to delete or send items.

This would work for photos, perhaps restricting it to an album or date range, but it could also work for handing your phone to a kid to play a game, or letting someone make a phone call.