Funny. Now when I checked some others in my discourse-repos I see that you’re right! But the first one I checked was discourse_docker, and it and discourse_api (and discourse-plugin-ci, which I think I, and perhaps everyone, should delete) still have master.
Reading your announcement again, I don’t see that you explicitly say that you did it for all the plugins and theme components (I think that’s what you mean by “all our branches”, but it wasn’t clear). Oh, and the first sentence refers explicitly to discourse/discourse, which I think also lead me to believe that you mean just discourse/discourse.