✅ Resolved
Using "edit post" and clicking "Admin tools" and then the box for "Collapse" marks a post: "This post is collapsed because of its low quality".
However, this seems to lead to buggy behavior. If one clicks on the title of such a post, one gets to a 404 page instead of the post page. Clicking on "(More)" switches it out to "Loading", but the rest of the post never loads.