@Sarah977
I also started hosting (properly) in 2016, so I'm guessing this was introduced later.
Definitely something for new hosts to watch out for. I am not adverse to the idea as almost everyone wants to get those first few bookings/reviews asap. However, the cumulative discounts can really bite one in the backside!
When I first listed a room in my house, I am absolutely sure I opted not to have Instant Booking because it just sounded like a really bad idea to me! I did leave on smart pricing, because Airbnb told me I would get on average £X for the room, and that sounded okay.
So, I completed the listing, published it and went to bed. In the morning, I woke up to be inundated with bookings at rock bottom prices. Smart pricing had halved my rates overnight and, somehow, IB had been turned on.
Well, that was a steep learning curve...