It is of course possible that the WiFi Assist on option is what iPhones always used to do and the option now available in settings allows you to turn this off which was not previously possible.

I have not read up on this so I could be completely wrong.

I don't think that's the case. My old phone would to totally tonto in a couple of locations when it couldn't work our whether a weak wifi signal was strong enough to use rather than the weak cell signal (or something like that - very hard to tell). It would started thrashing ands you could practically watch the battery count down. Took a restart to clear it once it started.

According to the news when it was released it was a new feature that people first noticed because they suddenly started eating into their data contracts - they would be sitting at home streaming video thinking they were on WiFi when in fact the phone was using mobile data to supplement the connection speed - that didn't happen before the last major update.

See

Money Saving Expert Comment

Vodafone Guidance

Just Google the term "WiFi Assist" to get lots of entries

I know, I remember when it was first being discussed...