From https://www.transportation.gov/briefing-room/biden-harris-ad... (linked in TFA):
The final rule improves the passenger experience by requiring refunds to be:
Automatic: Airlines must automatically issue refunds without passengers having to explicitly request them or jump through hoops.
Prompt: Airlines and ticket agents must issue refunds within seven business days of refunds becoming due for credit card purchases and 20 calendar days for other payment methods.
Cash or original form of payment: Airlines and ticket agents must provide refunds in cash or whatever original payment method the individual used to make the purchase, such as credit card or airline miles. Airlines may not substitute vouchers, travel credits, or other forms of compensation unless the passenger affirmatively chooses to accept alternative compensation.
Full amount: Airlines and ticket agents must provide full refunds of the ticket purchase price, minus the value of any portion of transportation already used. The refunds must include all government-imposed taxes and fees and airline-imposed fees, regardless of whether the taxes or fees are refundable to airlines.