I just noticed that search results now contain a new query string parameter named "s" with value in the format of
s=[a]|[B] where "a" is the search result index and "b" the relevance points, i.e., how relevant is the result to the search term.
For example, this search returns this as the first result:
What is the purpose of adding this to the URL?
When sorting the results by something else (not relevance) the value of "b" above becomes 0 for all the search results, while "a" is still the index.
Also I noticed this happens just here on MSE, so guess it's some sort of test(?).