suspending postal seems like a reasonable thing to believe.
This place had the story 10 hrs before CNN
https://www.businessinsider.com/usps-remove-mailboxes-several-major-cities-ahead-of-inauguration-2021-1
“As a security measure ahead of the January 20 event, mailboxes in at least 17 states including California and New Jersey, as well as Washington, DC, will be temporarily removed, several press releases from USPS said.”
“At least 14 post offices in DC will also be closed on Inauguration Day, the Postal Service said in a statement.”
I can't find any mention of it @ USPS
https://about.usps.com/newsroom/service-alerts/