Many games nowadays have DLC's, which brings me to this question. I'm not talking about free Pokémon events by going to a store or a free Pokémon by purchasing something, i'm talking about a real exclusive game enhancer that can be obtained digitally with money or Nintendo eShop.
MUAHAHAHA!
(Translation: silver was right in 2020)
As of December 14, 2023, two mainline and two spin-off Pokèmon games have DLCs. The mainline ones are: S/V (Scarlet and Violet) as well as SW/SH (Sword and Shield), and the spin-off ones are: Pokkén Tournament DX and Pokémon Mystery Dungeon: Gates to Infinity. Here are their names:
Mainline:
S/V: - The Teal Mask - The Indigo Disk
SW/SH: - The Isle of Armor - The Crown Tundra
spin-off:
Pokkén Tournement DX: - Pokkén Tournament DX Battle Pack
Pokémon Mystery Dungeon: Gates to Infinity: - Many dungeons bought as seperated DLCs listed here
NOVEMBER 2020 UPDATE
SURPRISE!!! Gen 8 has two lengthy DLCs called "The Isle of Armor" and "The Crowned Tundra", so as of today, yes there has been mainstream Pokemon DLC and there's a high chance they may continue with doing this.
I dont believe that any Pokemon game has had DLC before. Particularly main series ones. Take this with a grain of salt.
Edit: Well this comment aged like milk.