Minecraft wiki entry for Unimplemented_features, associated text: Unused features are still in the game but can't be obtained without using commands or third-party tools.
Mentioned features have been mentioned by Minecraft's developers but their implementation has not been confirmed yet.
Removed features once were in the game but since have been removed.
Planned features will be in the game in an upcoming version of Minecraft. They already have been released in a development version or their implementation has been confirmed.