Asking to borrow someone's phone is a common theft tactic. I wouldn't do it, for fear of someone thinking you're up to no good.

Burners / prepaid SIM's are still a thing.

I doubt RMS frequently borrows phones from strangers. It's usually from people who know him and are with him. They already have the phone, so RMS's using it doesn't increase the tracking it's already doing, and in some ways it pollutes the tracking data, also good.

I once suggested some alternatives to him like Meshtastic, but he travels too much to random places for that to be practical.