The coronavirus spreads through the expulsion of droplets from the nose and mouth. Blowing into a whistle entails, duh, expelling droplets from the mouth. Thus, the NFL has an optional replacement for the whistle this year.
Per a source with knowledge of the situation, officials will be given the option to use a hand-held electronic device that, when a button is pressed, will “blow” the whistle.
It’s not a mandatory replacement for the whistle. Those who choose to use a whistle would blow it in conjunction with a face covering, to minimize the spread of droplets.
It’s smart to make an electronic option available (as long as any inadvertent button-pressing can be kept to a minimum), and it’s another reason why football will look very different in 2020.