It used to open every door in Tibia. However, that is not the case anymore. Shapeshifter was the owner of this key and many other unique keys. Since Shapeshifter was hacked, his key collection (including key 0000) was lost. Other collectors have tracked down the hacker and bought the key collection to preserve its history and did so successfully. This key was added into the game by one of the map creators (before CipSoft became a big company they used volunteers to do areas). This person added the key into the game in secret. When CipSoft found out that there was a hidden key, they removed its function.
The old master key was wooden (brown) and the new one is magical (purple).
The Master key Wooden Key 0000 was created by CIP. It serves as best we can gleam to allow CIP members an easy way to access rooms that in all normal circumstances would never be opened. The first version on the game world Antica is a wooden key which after falling into player's hands was quickly deactivated. The most current version is a Magical Key seen on various test servers.