A list of all the previous Pokémon and their returning status has been posted on ResetEra. If this list is to be believed, then around 470 of the 809 existing will not be available in the Galar region, where the latest installment of the series takes place.
And there are a few very notable ones that are absent as well. For example, the Charmander line is the only returning line of starter Pokémon to make the cut. Mew, on the other hand, is the only existing Legendary Pokémon to return. None of the Fossil types will be in the upcoming eighth generation. Many pseudo-legendaries, Pokémon with high stats which make them competitive-viable, are also not part of the list of returning Pokémon. It still remains to be seen if this list is believable. If it is though, then it will definitely shape fans’ decision to buy Pokémon Sword and Shield. The absence of many starters and pseudo-legendaries will also likely shake up the competitive scene a lot more than most people expected. (Source: ResetEra via Polygon)