I've been on the subway (Metro) here in the DC area a handful of times in the last several months. In general, I see it as pretty safe. The stations are outside or indoor with high ceilings. The number of passengers is low -- I've never had a problem being at least 6 feet away from other people and often 10 feet. Masks are required and the rule (law) seems to be respected.
The busiest time was on the weekend. Traditional M-F commute times had sparse ridership since 90% plus of DC office workers are still working from home.
The rub is later when many more people return to the system.