The forum has always operated a dual function as a database and a discussion forum. My view is that, months or years later, marginal posts can be cleaned from historic topics if they clearly don't relate to the topic, are low quality. duplicates, misleading or misinformation. I also cull poor quality scans when newer scans of the same image is posted, fix up dead links, etc.
I don't really agree with deleting posts immediately after posting. Immediate removal or editing of posts should generally be done only when the content violates forum rules.
A forum will involve discussions that digress off topic. Split off-topic posts to a new topic if needed rather than deleting, especially if the discussion is still ongoing.
All moderators should generally adhere to this rule. I may add to the moderator guidelines.