I’ve implemented this as an excluded_categories setting: FEATURE: Add excluded categories setting - Pull Request #13 - VaperinaDEV/discourse-topic-preview-modal - GitHub
I initially considered making the behavior depend on where the topic is being viewed, but I think checking the topic’s actual category is the more consistent approach.
This means that if a category is excluded, its topics will always open as full topics rather than in the modal, whether they appear inside that category, on /latest, or in another topic list.
I think this also avoids the potentially confusing situation where the same topic could open in a modal in one place but as a full topic in another.