The ability to transfer ALL of your Pokémon to new games in the series has always been a constant since the beginning of the series until the new games were released on a new platform.
Pokémon Sword & Shield is the first game in the Pokémon series to break this constant. It was revealed during the Nintendo Treehouse at E3 2019 that the new games for the Nintendo Switch do not have National Dex and therefore you will not be able to download all the Pokémon that exist in previous games.
For context, in the Pokémon series, the National Dex is the complete list of all the Pokémon that exist. The list has been growing since the beginning of the series but has always been present in all new games in the series.
What this means is that who has all captured Pokémon (this is even the motto of the series, Gotta Catch ’em All) will be limited in the Pokémon that they can transfer to Pokémon Sword & Shield.
There are currently over 800 Pokémon and GameFreak has not yet confirmed how many of those Pokémon you can transfer to the new games.
The studio also said during the Treehouse of Nintendo at E3 2019 that the mechanics of Mega Evolution and Z-Moves were also not preserved in Sword & Shield.
The reason for GameFreak not to exist National Dex is that they have not had time to achieve the level of quality desired in the representation of each Pokémon.