Search is often used for finding articles to edit; the ability to exclude protected articles would make that more effective. GrowthExperiments, which offers articles with simple editing tasks to newcomers (and thus needs to avoid recommending protected articles), currently filters out protected articles on the client side (via action=query&prop=info&inprop=protection) which is far from ideal and makes proper handling of result sizes and offsets impossible.
It would be nice to have a CirrusSearch keyword (maybe hasprotection:edit / hasprotection:move?) for filtering for protection status. Page protection changes are accompanied by a null edit, which pushes status changes to the search index, so AIUI all that would be needed is to add a protection field to the ElasticSearch index, add it to the EventBus event for new revisions, and register the appropriate search feature.