This is a very good point. Let’s discuss an approach that we all can follow.
Discourse offers many options to organize content, including features to process posts automatically. Separate topics and posts can be merged in a single topic, they can be closed, unlisted, archived, and deleted. Confused? Yeah, it can be confusing. 
But this is a simple way to look at this:
- Unlisting is an exceptional measure. Something is going on with a recent topic or post and you want to remove it from visible places while dealing with it.Those with the link still can access it normally.
- Close → Archive → Delete is a logical sequence. To decide where a piece of content belongs we need to ask: how useful is this content for the forum users today and in the future?
We could start with something like this:
- For announcements and updates, try to have topics updated over time instead of starting new topics for each update. This is also good because topics get more readers, and then Discourse’s notifications and algorithms promote the established topics better as long as they receive updates, comments, reactions, and views.
- When an event ends, do some cleaning, merging, closing, and archiving topics as needed.
- In event categories, consider setting automatic timers by default i.e. “Close topic after 3 months without replies.”