Mostly hypothetical, but in theory we could end up with articles that don't get deleted from the index. I'm thinking mainly in the situation where you start over on a wiki but don't prune the Elasticsearch index--that is: you had a wiki named foowiki, stopped, and are starting foowiki again. The old articles will never be pruned since nobody can delete non-existing articles :)
When we get such a result we already don't display it (since we check for page existence application side after fetching our results), so it shouldn't be too terribly hard to insert a low-priority delete job around that point.