This is only being looked at as a possibility at this point, all while being aware of not wanting to see testing resources unnecessarily wasted on people engaged in "non-essential" flying around.
Adding a requirement for a negative test within 5/6/7 days of domestic flight departure can be made to work to reduce the spread of the virus, if the objective of such requirement is primarily to drive down the demand for less-than-fully-"essential" travel -- and that is even if taking it as a given that a test requirement of that sort wouldn't make more than a 1-2% decrease in the chance of a traveler spreading/getting an infection.
The way to really drive down non-essential air travel is to have a 24 hour test requirement before interstate domestic flight. Add the individual's cost in time and money of being tested and the risk of that individual being a positive case to the domestic air travel picture, and there will be a precipitous drop in non-essential flight travel by that individual and many others like that person.