This is one of the biggest food issues for airlines.
Whilst some will make announcements asking people not to consume nuts if the crew are made aware of pax onboard with an allergy, they can, in no way guarantee that the cabin is nut/nut trace free. The person sat in that pax seat only the previous flight could have smothered the thing in traces of nut oil etc from eating their own snacks.
It also becomes a big issue when the product served to everyone else contains nuts or is nuts. Most airlines as a nibble with drinks in at least Business and First supply some sort of nuts with a drinks service. Are they supposed to then remove part of the product that 40/50/60+ other people have paid for as part of their ticket and service provision, to satisfy the requirements of 1 other passenger? And not saying that the allergy is not a serious thing, but I do wonder what the airlines would have to do to placate the other passengers, how much cost that would add, and how that would be built into everyone else fare?
It is a total mine field for airlines.