Update: During E3 2019, we had a chance to speak to Pokemon Sword and Shield produce Junichi Masuda and director Shigeru Ohmori about the upcoming games. They confirmed that Sword and Shield don't have random encounters in the traditional sense. While you won't see every Pokemon in the overworld--sometimes there will be an exclamation point indicating that a Pokemon is hiding--Pokemon won't just randomly attack you from the tall grass with no warning like in previous games. Original story follows.
The Pokemon Direct that aired on June 5 showed off a new trailer and more details for Pokemon Sword and Shield. One of those new details was a bit unclear; the trailer and subsequent clips showed Pokemon roaming the overworld, similar to Let's Go Pikachu and Eevee, but we also saw a random-style encounter in the tall grass. So, are there going to be random encounters or not?
From what we can tell so far, it seems like random encounters are mostly, if not totally, gone. The Direct showed a variety of Pokemon roaming the overworld, much like in the Let's Go games. You encounter these wild Pokemon in something called the Wild Area, which refers to the expanse of nature between cities; in the Wild Area, you'll encounter different Pokemon based on your specific location and the weather. You initiate battles with these Pokemon by running into them, though some will chase you down once they spot you.
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The Direct also showed a more typical wild Pokemon encounter in the tall grass. As it turns out, you have some control over these encounters as well. A small exclamation point will appear in the grass, indicating that a Pokemon is there, and then you can initiate the battle by running into it--just like with the Pokemon that physically appear in the overworld. It's unclear if there are any occasions where Pokemon will attack you totally at random and as a surprise, but it seems like that's unlikely. We've reached out for clarification and will update this story once we hear more.
For more news from the Pokemon Direct, see our explanation of the new Dynamax mechanic and all the new Galar Pokemon. Pokemon Sword and Shield are set to release on November 15; you can learn more about that in our pre-order guide.