I think a lot depends on how you think of Pinned.
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If you think of it like last century’s forums, you might think Pinned would be a kind of “sticky post” and always listed first.
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If you think of it like Discourse, Pinned is more like should be read at least once.
True, if Pinned is used more as an “easy to find reference topic”, having them automatically unpinned after reading doesn’t help. If members want to keep it readily available they need to use the topics Pin button or Bookmark the topic… or be good at using search.
in:pinned
The problem with Pinned topics is that they occupy valuable real estate – the top of every topic list! If you have too many of them, they permanently shift more active topics down the list.
If the unPin when read isn’t a good fit for a forum there is a site setting to disable it from happening. No “default” will ever suit every forum. That’s why they’re options.