Pokemon Brilliant Diamond and Shining Pearl will not have Pokemon from newer games. Last week, Nintendo France made commented on Facebook that Pokemon Brilliant Diamond and Shining Pearl would only include Pokemon "up to the 4th generation." That means that the new Pokemon games will be the second consecutive set of games not to feature the full National Pokedex and its 900+ species of Pokemon. The comment was later deleted and should obviously be taken with a grain of salt since it didn't come directly from The Pokemon Company (which is a separate entity from Nintendo or its international affiliates) but it still seems like a rather obvious warning to keep expectations for the upcoming game tempered.
The news that Pokemon Brilliant Diamond and Shining Pearl won't have every possible isn't exactly a surprise given that the games were billed as "faithful remakes" of the original Pokemon Diamond and Pearl games. While The Pokemon Company has added over 400 new species of Pokemon to the franchise since the release of Pokemon Diamond and Pearl 15 years ago, it still would throw off the nostalgia of revisiting the Sinnoh region if you could do so with a ton of newer Pokemon at your beck and call.
However, there is precedent of a Pokemon remake featuring Pokemon that were available in the original games. Pokemon Omega Ruby and Alpha Sapphire famously included hundreds of Pokemon that didn't originally appear in their Gen 3 predecessors, as did Pokemon HeartGold and SoulSilver. Even the more recent Pokemon: Let's Go games featured Alolan regional variant Pokemon that obviously didn't appear in Pokemon Red and Blue.
Still, The Pokemon Company has moved away from the model of releasing Pokemon games that include compatibility with every single Pokemon species, and it stands to reason that Pokemon Brilliant Diamond and Shining Pearl would be limited in what Pokemon species can be captured in the game. The real question is how Pokemon Brilliant Diamond and Shining Pearl will sync up with other Pokemon games like Pokemon Sword and Shield or the upcoming Pokemon Legends: Arceus.
Pokemon Brilliant Diamond and Shining Pearl will be released on November 19, 2021.