Owners of iPhones (and iPads with LTE) have always had the option to disable Wi-Fi Assist, although the setting isn't in the most intuitive place.
You won't find it in the Wi-Fi settings, but instead at the very bottom of the Cellular options list. And Apple turns it on by default.
Just like the statistics provided by iOS for cellular usage, the Wi-Fi Assist data usage figure can be reset manually at the beginning of a customer's phone service billing cycle.
While it won't prevent your data usage from skyrocketing without you knowing, at least it gives you a feel for any overage impact.
For its part, Apple says Wi-Fi Assist really shouldn't have a major impact - although some would disagree - because it doesn't switch to cellular data for bandwidth intensive activities. Wi-Fi Assist won't offload to cellular when roaming, for example, nor for downloading background content in apps or streaming audio / video content in third-party apps.