Per their company spokesperson:
As of June 1, 2013, passengers who report a peanut allergy to their reservations or gate agent at the airport will not have peanuts(or products containing peanuts)served onboard their flight.
While Delta cannot guarantee a peanut-free environment, whenever possible Airport Customer Service agents will notify Flight Leaders and Pursers during their pre-flight briefings.
Flight attendants will make an appropriate onboard announcement to let customers know that peanut products will not be served.
This removes the “three-seat” buffer area that was previously offered for passengers with a peanut allergy. This new policy only applies to peanuts at this time and does not include tree nuts.
“As for why we did it, it was a combination of feedback from customers, combined with creating the most efficient operational approach to meeting the process,” said Black.