Recently I came across this phrase:
Why is 関係した in the past and not in the present? In this case, is there any difference between them?
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Consider the English verb relate. We commonly use the past-tense form as an adjective.
things related to blood type
This is one of those uncommon areas where the Japanese and English grammar line up -- likely due to the shared underlying nature of the concept. The relation itself is something that is already established, so we use the past tense (or completed aspect, if we're being more specific).