Sorry, I didn’t want to overwhelm with options. 
If the problem is clutter created by obsolete posts that didn’t stir discussion and won’t stir it, then archiving these topics is a good default option. Let me archive Delay of the Board of Trustees election so we can see what this looks like.
Merging single-topic announcement posts in a single “Archive” topic is a good idea. However, I don’t think that the best criteria is “announcements by the Foundation” and the best solution is to mere them all in a single topic. Instead, I suggest we create these Archive topics in any category needing them, and then we archive there any obsolete announcements. This way that information can still be found at a category level (i.e. imagine someone wants to check everything about #identify-topics-for-impact including archived posts). Nowadays the Foundation is th main poster of these announcements but hopefully there will be other posters as more groups get involved in MS implementation of recommendations and adopt the forum for their communications.