From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon Platinum, like other solitary versions of the main series games, contains less pre-release information because it is based on the previous paired versions of the generation. Likewise, few things changed in comparison to Pokémon Diamond and Pearl.
The Battle Park from Pokémon Diamond and Pearl is present in the internal data of Pokémon Platinum but it can only be accessed by modifying the saved game data or through cheating. This indicates that the Battle Frontier was created some time after the development of Platinum started.
An earlier sprite for Giratina's Origin Forme appeared in several pre-release screenshots. An unused animation frame for the final sprite has also been found in the internal data. Unused sets of sprites for Shaymin's Land Forme as well as prototype sprites for the items which change Rotom's forms are also present.
In addition to Pokémon Platinum's exclusive tracks, all unused tracks found in the internal data of Pokémon Diamond and Pearl are also present.
The unused items discovered in the internal data of Pokémon Diamond and Pearl are also available in Pokémon Platinum, alongside its own set of exclusive items. However, two finalized and used items from Diamond and Pearl that have no use in Platinum still remain in the game's programming code.
Leftover dialogue within the internal data provides further details on how the game developers might have debugged the game. For instance, such text refers to features such as a Sound test and an option to modify the real-time clock (RTC).