Anything longstanding has been around for a long time. If it's a longstanding tradition for your school's senior class to play a prank, that means it was even going on back when your grandfather's class let hundreds of crickets loose in the halls.
Humans keeping cats as pets is a longstanding custom, and the habit of cooking food before eating it is also longstanding. You can call yourself a longstanding customer of your town's last remaining video store if you've been going there for years, or a longstanding fan of your favorite tennis player if you've followed her entire career. The adjective longstanding has been around since the early 1800s, so it's longstanding too.