From Minecraft Wiki
Unimplemented features are features which aren't naturally obtainable in the game.
- 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.