The Disk of returning was used to let players exit the Black Hole in RuneScape Classic. At the time a non-player character called Thordur sold them for 10 coins, while now they sell for an astounding 4,317,363 coins. This item is the oldest rare or discontinued item in the game beating out the pumpkin as this disk was created back in July 2001.
People that were banned were put into a hole and trapped there until their ban was over. A lot of people got themselves banned just to see the hole, so Jagex made a mock hole and created a disk to let players out once they went in. These items are high in numbers but only a few people have them. These disks cannot teleport players in or out of the Black Hole as the Black Hole no longer exists.
It was removed because a lot of banned people convinced some people who were visiting the black hole to drop their disks, because something "cool" would happen. This was a scam that would trap those players in the black hole and allow the scammers to escape.