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.
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.