The Battle Frontier is a recurring postgame area in many Pokémon games that offers players the chance to fight high-level Pokémon trainers. In Pokémon’s fourth generation, Diamond and Pearl didn’t feature a Battle Frontier, but Platinum did. Given that Brilliant Diamond and Shining Pearl are remakes of the Generation IV games, some in the community speculated prior to the remakes’ launches that the Battle Frontier could be featured in the remakes.
However, like many other elements exclusive to Platinum, such as the character of Looker, the Battle Frontier is not present in the remakes. Instead, you can visit the Battle Tower, which was previously featured in Diamond and Pearl’s original releases and was later made into a section of the Battle Frontier in Platinum. At the tower, you can face off against multiple trainers in a row while earning Battle Points, which you can use to purchase various items at the Battle Tower’s shop.
The Battle Frontier’s exclusion can likely be chalked up to the area not being present in the original releases of Diamond and Pearl. This omission might come as a disappointment to some, but there are still quite a few things to do postgame, such as earning/completing the National Pokédex, catching Legendary Pokémon in Ramanas Park, and rematching the game’s Gym Leaders.