I wish the term used was “water resistance” and not “waterproof”. That semantic annoys me.

In my experience they are waterproof. I have AppleCare+ so if my phone were to suffer water damage it'd be relatively cheap to get it fixed, and I fairly frequently expose my phones to water. I've never lost one to water damage so far.

My understanding is the water proof glue can in theory be damaged if it's exposed to extreme temperatures and the like - that's why Apple doesn't cover water damage for free. But I've done that too (I've had my phone shut down with a temperature error a few times - usually when it's on my motorbike in the full sun for an extended period of time with a map open... it's fine while riding with the windchill but if you park the bike or get stuck in traffic, then it can overheat). Mine still hasn't died int he rain including 60mph driving rain (on the motorbike...) which it's supposedly not designed to handle. Every time that's happened I haven't really had a choice. I'd rather get where I'm going than stand in the rain parked on the side of the road waiting how long, five minutes? three hours? for the rain to stop.

The one time I have lost a phone to the water damage it wasn't water it was extreme humidity. So I suspect putting the phone in a backpack, and then riding a motorbike in the rain, will actually increase the risk. Proper water has surface tension that stops it entering microscopic gaps. Humidity doesn't do that. And the inside of a backpack is never dry on a motorbike in the rain.