In iOS 9, Apple introduced Wi-Fi Assist, a theoretically good feature that defaults to cellular data when your wi-fi connection is sucking. Unfortunately, in doing so, it quietly went through my data plan without telling me. In the latest beta of iOS, Apple is fixing that -- kind of.
Wi-Fi Assist is still present in iOS 9.3 beta 1; the difference is a tiny counter underneath the toggle to enable Wi-Fi Assist, which tells you how much data has been used by the feature.
It's not a perfect fix -- I would still like some kind of symbol in the status bar (or a drop-down notification) when cellular data is being used in preference of wi-fi. But at least with this counter, I'll be able to see if iOS is really the culprit for me running over my data allowance, rather than just guessing.