This is for all themes, not just Horizon.
Our thinking has changed here a little — blocking the New Topic button when viewing a read-only category is making an assumption that the user wants to create a topic within that category, but that may not be true. They may just happen to be looking at a read-only category before creating a topic under a different category.
When the category they’re viewing isn’t read-only, we try to help by pre-populating the viewed category. When the category they’re viewing is read-only, we can’t do that, so we’re just leaving it blank. If the user tries to select the read-only category, they will see that it’s disabled with a “read-only” label.