This is an extension of a temporary measure announced earlier this year, a measure with clear parameters as the State Dept. works with the Department of Homeland Security as well as the airlines to help facilitate travel back to the U.S.
Citizens can travel back to the U.S. from abroad ONLY on a passport that expired on or after January 1, 2020, just before the pandemic hit. Any passport that expired prior to that date is not valid.
Travel on the expired passport to the U.S. MUST be completed by March 31, 2022.
Travel can ONLY be for a return to the United States. An expired passport can not be used to travel to another international destination or to stay in another country longer than it takes to make a connecting flight back to the U.S.
Airline travelers must also show a negative COVID-19 test, regardless of vaccination status, in order to fly.