Just like the following:
Link to their site: Top SMS tools and hotlines - Apps4Flip And WebAdb Forums
There is a feature topic based on that Flarum feature Lock topic content for logged out users.
There are also Allow to hide part of a post content with "login to see" or "reply to see" function and Ability to hide content from subsets of users using BBCode.
But I think so far no one has created a plugin or paid someone to create one.