> Isn’t a sterilized metal can safe by itself?

No, because of reactions between the contents (particularly acidic contents) and the can.

That does make sense. Is this a newer thing, or has it been this way for a long while?

The can of beans on the fire is a camping archetype - one I've witnessed people mimic first hand. Seems unhealthy, knowing this now.

Not sure.

I've witnessed people camping, placing an opened can of beans directly on the fire and such.

Seems absurd now, knowing plastic is in the can!

If anyone else knows the reason for the lining, I'd be very interested in learning more.