Well, normally when you buy airline tickets with Delta Air lines they don't expire easily, they expire one year after purchase. However, to allow for greater travel flexibility, all tickets expiring in 2022 and all new tickets purchased in the same year can be rescheduled from December 31, 2023 to 2024. 

Due to COVID-19, mandatory mask wearing was essential for any type of travel; however, now that the disease is no longer a danger, masks are no longer required for domestic travel and on most international flights. If you have further questions, you should go check the mask rules with Delta airlines. Although the mandate is no longer mandatory, you are often required to bring a mask as the country you are visiting may require you to wear one. 

Delta Air Lines planes are thoroughly cleaned by an in-house cleaning team. They hand-clean everything on the aircraft, from seat surfaces, consoles, TV screens, windows, doors, lavatories and other areas that people use a lot while traveling.