In the international signal flag bag this is the letter Q. Nowadays a ship entering port and declaring it has NO communicable diseases and requesting free pratique flies this flag. In other words, the original meaning has been stood on its head.
Nowadays if a ship wants to declare that it has a disease onboard it flies this flag.
This is the letter L in the flag bag. I have no idea why they decided to use this instead of the original quarantine flag but there you have it. So, if you have the corona virus help out your neighbors and those nice people from the Jehovah's Witnesses who like to knock on your door and earbash you and fly your quarantine flag. Or, you know, print out the L flag and tape it to your door. I assure you everyone who sees it will be mystified.