From Delta.com
Mask Exception Process
Delta is committed to requiring all customers and employees to wear a face covering as a consistent layer of protection across all Delta touchpoints. As of July 20, any customer who believes they have an underlying condition that prevents them from wearing a mask is required to complete a "Clearance-to-Fly" process before being permitted to travel, for everyone's safety. The virtual consultation process:
Could take up to an hour, so customers needing a rare exemption should plan to arrive at the airport early
* Will be facilitated by a Delta agent with a third-party medical professional
* Will be required for each individual trip
So a Doctor's note will have a value of close to Zero and only the Delta Doctor's opinion will be the deciding factor..