Just found out that a Linux machine connected to a USB printer can be used as an AirPrint server.
This may not be news for some people. But if you have a Linux machine connected to a USB printer, and go to
Administration – Printers – Server – Settings
and check off this option
“Publish shared printers connected to this system”
Then any iOS device on the same Wifi network will see the printers attached to this machine via USB available in AirPrint.
I was just able to print from my iPad to an HP LaserJet 1020 that is almost 15 years old and neither Wifi nor AirPrint capable.