I think sitemaps might help - firstly to help make sure crawlers are aware of newer versions, and secondly we could assign priorities so that the newest versions get the highest priority, and older versions have their priorities drop as newer versions are uploaded. https://github.com/purescript/pursuit/issues/106
Come to think of it, adding a link to the most recent version of the docs could also help boost the search rankings of the most recent version relative to other versions, as “number of times this page is linked to” is one of the main heuristics search engines use, as I understand it (although this information might be out of date by now, who knows).