They take them off the books. They aren’t counted as part of the official debt, and they’ll never be repaid. It’s not like the Fed is ever audited or can go broke.
Off what books? I can look up bonds held by the Fed in less than a minute.
They aren’t counted as part of the official debt,
Sure they are.
and they’ll never be repaid.
When they mature, the Treasury pays the Fed. When interest is due, the Treasury pays the Fed. It happens all the time.
It’s not like the Fed is ever audited
Except for the annual audit they publish every year.
or can go broke.
That's true, a central bank can't go broke.