Not Provided is about to expand when Bing goes HTTPS over the summer, ultimately blocking query data in the referrer path.
Bing announced they will begin encrypting their searches over TLS. So instead of seeing referrals from http://www.bing.com, they will begin coming from https://www.bing.com.
This happens this summer and with the change, Bing will no longer pass query data to webmasters. The [not provided] saga will be expanded from Google to Bing, where marketers won’t be able to know in detail how searchers are finding their sites through the top two leading search engines…
Bing To Encrypt Searches By Default & Referrer Data To Go Not Provided This Summer