There is a league rule on how much the ball is to be inflated. That nullifies any need to send a written policy/memo that deflating the football isn't allowed.
Rule 2 The Ball
Section 1
BALL DIMENSIONS
The Ball must be a “Wilson,” hand selected, bearing the signature of the Commissioner of the League, Roger Goodell.
The ball shall be made up of an inflated (12 1/2 to 13 1/2 pounds) urethane bladder enclosed in a pebble grained, leather case (natural tan color) without corrugations of any kind. It shall have the form of a prolate spheroid and the size and weight shall be: long axis, 11 to 11 1/4 inches; long circumference, 28 to 28 1/2 inches; short circumference, 21 to 21 1/4 inches; weight, 14 to 15 ounces.
The Referee shall be the sole judge as to whether all balls offered for play comply with these specifications. A pump is to be furnished by the home club, and the balls shall remain under the supervision of the Referee until they are delivered to the ball attendant just prior to the start of the game.