This includes items and quests that can no longer be obtained.
The in-game information in this article is kept purely for historical purposes and should probably not be under any other categories.
The keyring took the form of a sixth bag slot which held a number of keys, most usefully dungeon keys among other keys, with dynamic capacity. Not all keys or items with key in their names are compatible with the keyring.
When a player first acquired a key, they would automatically acquire a keyring that would hold compatible keys. The keyring sized dynamically for the number of keys in possession, up to 32 keys.
The keyring is the leftmost button on the bag bar.
The keyring appeared as a thin key icon between the leftmost pack and the micro menu. Clicking this revealed a bag similar to a normal container.
Right-clicking a key in your bank moved that key into your a normal inventory bag slot.
The keyring would adjust the number of slots to always accommodate all keys. If it appeared that you had filled the bag, close and re-open it. It would expand until there are 32 total slots filled and then it would not expand anymore.
The keyring button was removed from the Interface Bar around Patch 4.2 build 14133 to make room for the Dungeon Journal button. A blue post effectively made it official that the keyring is going away.[1]
The tips as you login still mention the keyring as of Patch 4.2.2, even though it was removed from the game.
On the PTR, it could be brought up if you give it a keybinding, but all the keys in it would try to move to your bags and you can't put them back, so the keyring was effectively broken. Presumably even the keybinding method to bring it up will be removed when the keyring is fully removed.
Excess keys (not obsolete nor useless) will try to move to your regular bags. If there is no room they will stay in a "backlogged storage system". So phantom keys could start filling up your free bag space.
Bashiok said, "it is our goal to make sure the removal of the Keyring causes as little inconvenience to you as possible."[1] However, almost no one in the player population agreed this was a result.
Patch 4.2.0 (28-Jun-2011): Removed. Existing keys either converted to gold, removed entirely, or moved to players' bags. If there was no room, hidden queue space would be used.