why does a search for "indicators" return the #1 hit as a closed thread with one upvote that was closed because it was an exact duplicate of the thread "List of application indicators" that has been upvoted 171 times, instead of that thread?
Closed questions may contain relevant content. For instance if a question is closed as a duplicate both the duplicate and the original question (linked from the duplicate) may contain relevant content.
I don't know what the algorithm for relevance sorted searches is but the tooltip says "Search results with best match to search terms". So sorting by relevance is about finding posts that post matches the search terms best and then it's irrelevant whether or not it's closed.
If you want to posts with most votes you can sort search results by Votes (and you can also sort by Newest and Active).