It's been around since iOS 9 but it doesn't use much data at all.
For example, my boyfriend has had the feature on the entire life of his iPhone and since June 2015 (this feature was introduced in September 2015) it's only used about 700 MB of data.
Other things that come into play include his wifi connection at home sucking, and his overall cellular data usage since June 2015 is well over 230 GB
It only kicks in when you have poor wifi and prevents pages / things from not loading by using your cellular connection as a last resort type of deal, and I'm sure there are measures in place that keep it from burning through your data / loading big files.
It's really not bad or a big deal.