I have flown millions of miles on US airlines and never heard this once. Peanuts can be found in chocolate bars, energy bars, trail mix, granola, popcorn products, Vietnamese spring rolls. Is the expectation that people who bring food products with peanuts on board not be able to eat them if someone declares their allergy? Is there any governing law on this or is it covered in the contract of carriage or similar?

Even when an announcement is made, there are bound to be people who don't hear it or observe it. That is why I am asking how people with this type of allergy prepare. Relying on fellow passenger compliance seems risky.