On a related note, car rental places now email me the contract and rental documents by default, I receive no paperwork unless I remember to ask. The problem didn't register with me right away (It was so similar to what hotels do when you travel, all paperless now) but the last time I returned the car I asked them what I was supposed to do if I was stopped for a traffic violation. "You can open the email from us on your phone and give it to the officer" they suggested. I recommend everyone ask for a printed contract instead.

As an aside, on iOS, the airline apps I have used work like the Wallet app and are visible on the lock-screen, the device must be powered on but remains locked when you display your boarding pass, etc.

I’ve been using mobile boarding passes exclusively for the past few years and it doesn’t even remotely work in the way you describe (within the US, at least; cannot comment on CBP).

1. There is no special app you need for mobile boarding passes. It has always been either a PDF or a PNG file emailed to you.

2. I was never asked to hand over my phone to TSA agents at any point. They just ask you to put your phone with the boarding pass QR code displayed over a QR scanner. At no point the phone leaves your hands.