When I follow the Related Questions section, I often come across closed questions. I would think that closed questions wouldn't be shown, since they aren't relevant.
Shouldn't closed questions be removed from the Related Questions list?
A question that is closed is not always irrelevant. Since there is always the potential for it to get reopened at any time, it makes sense for them to still linger. 10k users can vote to delete something that is closed after 24 hours. They can also vote to undelete any deleted content as well.
If something is deleted it won't show up anymore, but closed still do. I agree with this current behavior.
The point of keeping closed questions around (rather than deleting them) is that they might still be relevant. For example, if a question is closed as a duplicate, it might still have phrased the question in a completely different way, and that different phrasing might be very useful to someone. In particular, looking at a related closed question might be just the thing you need to realize that the question you're looking at solves your problem.
I know this is an old question, but I just want to reiterate that I think showing Closed questions in the "Related" sidebar is silly. I just clicked on one, and it had been closed for being a duplicate. There were no answers. Ideally it should be deleted, since it serves zero purpose. But at the very least, a "duplicate" closed question shouldn't show in that list!
I agree that closed questions could appear in Related sidebar, but they should be clearly marked as [closed] at the end of description, the same way as they shown in search results.
Could they also have less rank in "related" algorithm to be not shown on the top?
One thing closed questions sometimes have is good answers. If two people ask essentially the same question, each question might have a good answer that's worth keeping. It is possible for moderators to merge the answers into one question, but that's not automatically done.
This being "by-design" makes no sense to me...Anton and TheTxi make good points about the closed questions still being valuable...but in that case, I don't think they should be closed.