Are armrests required by regulation to be down and locked during takeoff & landing?
With regard to aisle armrests, there is a requirement related to armrests that must be movable, but does not specify that it must be down for takeoff and landing.
Regarding armrests down for takeoff and landing, the recommended practice is in part due to known issues with certain seats that armrests should be down for takeoff and landing. See 8900.1 Volume 3 Chapter 33 Section 3, paragraph 3‐3483 Protruding Passenger Seat Armrests.
While it is not a regulatory requirement for armrests to be down for takeoff (actually, they don’t lock), it is a recommended best practice. If it’s in the flight attendant manual to verify all armrests down for takeoff and landing, then flight attendants should verify they are down when performing the cabin check as it is their company policy and is safety-related.