key performance indicator" for search is the number of people who get zero results for a search. The objective is to make the number of people who do as small as possible. 
In the early days of the Dutch Wikipedia, a librarian was always happy to explain how he improved search results at his library.
His first observation was that he needed to know what people could not find. The observations were aggregated in timeslots. In this way he knew what people were looking for. His favourite observation was "People are stupid; they do not know how to spell". Allowing for the most prevalent spelling errors improved the results a lot. The other part was that people were looking for things the library did not provide.
The message for the Wikimedia search team.. Consider publishing the known errors aggregated over time. Have the community mark the spelling errors as such and use that to serve content anyway. The other part where we do not have data, consider that Wikidata has more information than any Wikipedia, when results do not exist as articles, publish what people seek and there might be a community out there adding missing articles.