Of course!
These are the most useful plugins. They are not official and we cannot guarantee that they are maintained by their authors in the long run:
- Advertise activity in a private category (discourse-category-lockdown)
- Lock topics behind a "crypto" paywall
An alternative solution is to use private categories for members only and public for other users to entice them to pay a subscription to read protected content. It can be done using the Discourse Subscriptions official plugin or, if a WordPress main site exist, with the Memberful plugin and other simlilar services.