I've been over at meta.discourse.org looking for reasons to explain why topics will resurface. It seems to be a deliberate feature of Discourse. It's a bit like supermarkets rotating goods on the shelf, they bring the older items to the front of the shelf so that they can be consumed.
To prevent this problem we have to close topics. That is, we have to actively go to a topic and press the "closed" button. This is a job that will have to be done by the community and moderation team.
Thank you @Malcolm for your effort to inquire about the cause of this happening.
I've seen other discourse forums use a feature to automatically close topics n days after the most recent comment. Might that work?
@Malcolm, @Torsten, @planteg, @Bobino, @Cecile,
I suggest we talk about this at the team meeting this Wednesday. We recently had a discussion and made policy decisions regarding topic locking. It may need to be revised.
Indeed this feature exists. However its implementation is rather indiscriminating if at the channel level. And this time based or responses activity base closing might close topics that are still worth keeping open.
We shall discuss this at the team meeting but meanwhile everyone feel welcome to chime in on your preferences.
For my part, I think that time sensitive topics like the one Udo stumbled on are very few so I have no objection to manually close them when they become irrelevant.
Some topics should be kept open so that people can add to them as new information comes in.
Other topics should be closed, for example, if they are rants and everyone has had a chance to vent.
My 2 satoshis
90 days is the default for that action but it seems that there are other factors at play.