The upcoming game Pokémon Legends: Arceus is set in the distant past of the Pokémon world, which means that some Pokémon cannot appear, as they don't exist yet. The concept of artificial Pokémon has existed since the early days of the series, as Pokémon Red and Blue featured technology that is more advanced than in the real world.
The Sinnoh region is coming back in a big way, as there are three upcoming Pokémon games that will use it as a setting. Pokémon Brilliant Diamond and Shining Pearl are straight remakes of the original Generation IV games, while Pokémon Legends: Arceus is an open world RPG in the style of The Legend of Zelda: Breath of the Wild. Pokémon Legends: Arceus is set in the distant past of the Sinnoh setting, where the Poké Balls are made of wood and powered by steam, and there is only a single village in the region.
The gameplay footage of Pokémon Legends: Arceus that has been revealed so far has shown some of the Pokémon that will appear in the game, but there are some that cannot appear due to timeline reasons. These are specific Pokémon created by technology that doesn't exist in the old Sinnoh region. Fossil Pokémon technically can exist, as Kabuto's Pokédex entries have claimed that living specimens have been encountered in the wild, so it's possible that some Fossil Pokémon still wander the land.
Mewtwo Won't Be In Pokémon Legends: Arceus
Mewtwo is a relatively new creation in the Pokémon world. The notes in the Pokémon Mansion in Pokémon Red and Blue explain the hi-tech process behind Mewtwo's creation, while its Pokédex entries talk about the gene-splicing method used to breed it. Mewtwo's exact age is unclear, but the technology to create it is far too advanced for Pokémon Legends: Arceus.
The Porygon Line Won't Be In Pokémon Legends: Arceus
Porygon and its evolutions are made from computer data, so the technology to develop them doesn't exist in the Pokémon Legends: Arceus world.
Genesect Won't Be In Pokémon Legends: Arceus
Genesect was created by Team Plasma and it's a modified version of a Pokémon that existed millions of years ago.
It's unlikely that Team Plasma exists in Pokémon Legends: Arceus time period, and even if it did, the technology to create Genesect wouldn't exist yet.
Voltorb & Electrode Won't Be In Pokémon Legends: Arceus
It's unlikely that Voltorb or Electrode will appear in their current form in Pokémon Legends: Arceus, as their design and typing is based on the modern Poké Balls. It's possible that there will be a Sinnoh form for these Pokémon that have a different type.
Castform Won't Be In Pokémon Legends: Arceus
Castform was created using advanced technology from the Weather Institute in Hoenn, putting it in a similar category to Porygon.
Type: Null & Silvally Won't Be In Pokémon Legends: Arceus
Type: Null was created by Faba of the Aether Foundation. Pokémon Legends: Arceus is set long before he was born.
However, Time, Space, & Dimensions Are Meaningless
It bears mentioning that there are some Legendary and Mythical Pokémon with the ability to manipulate time (Celebi), space (Palkia), and reality (Hoopa). The mascot of Pokémon Legends: Arceus is a godlike being. As such, it's possible that these Pokémon can use their incredible powers to bring artificial Pokémon back from the future for some awesome boss battles.
Pokémon Legends: Arceus will be released for Nintendo Switch in 2022.