I admit I don’t understand why some would want to have teasers. Most want search engines to be able to find content. (Note - Google doesn’t log in)
Anyway, in terms of permissions there are these in Discourse:
login required - if a visitor is not logged in (which requires being Registered), they can’t see anything
List pages display Category titles and Topic titles.
Categories set permissions based on Group membership, where permissions can be set for
If a visitor is not in a Group that has at least “See” permission, they will not see the category title in any lists, nor will they see any topic titles for topics in the category or any posts made in topics in the category.
Hidden - if a topic is hidden it can be accessed only by its URL. Its title will not display in any lists.
Hidden - Posts can be hidden, but IMHO it’s more like “spoiler” than hidden.
There currently is no way to
"Show Category title but not Topic titles"
"Show Topic titles but not posts"
unless you post about them somewhere that can be seen.