The American Express SFO Centurion is temporarily closed, the American Express app states this but the online landing page does not. Reddit user awesome2000 confirms that the lounge is in fact closed in person with a note stating:
The San Francisco Centurion Lounge is temporarily closed due to maintenance issue. Repairs are underway and the lounge will reopen as soon as they are completed. Sorry for the inconvenience.
It was also closed yesterday, but unsure how long it’s been closed or what the maintenance issue is.