Some of the communities that closed down in response to the API changes explicitly shifted to Discord

Sigh

Legitimately one of the most annoying things out of all of this is how so many tech writers have no idea what they're talking about when it comes to explaining the feelings and behavior of reddit communities. Like they don't ever seem to interview anyone or ask direct questions to understand what is happening, they just take a cursory glance at comment threads and pinned posts, and assume they understand what's going on. I can't count how many articles I have read that get the facts about this whole situation right but still seem to completely misunderstand it.

They did not "shift" to Discord, they used already established Discord channels in lieu of the subreddit, until the subreddit came back.

Many subs already had parallel discord channels, but users don't use one or the other, they use both. Because Discord is a fundamentally different kind of platform. It's exactly the same way that many forums back in the day would also have chat rooms attached. Same community, using two different methods of online communication, at the same time, for different purposes. No one would ever have suggested IRC was equatable to a forum.

All that happened was when the subs closed, users congregated in the Discord to stay connected. It was never going to be a new permanent home. They were not seeking a new home, that's the critical part. I've seen very, very few people suggest Discord as a permanent replacement and if they are it gets shot down. They were simply waiting out the protest on Discord until they could go back to Reddit or, if an alternative showed up, go there. It was a bomb shelter.