Re: I don't get what the problem is...
The problem is that if designers follow current best practice, which in part derives from the kludge than Apple introduced with the I-Phone then the screen size will be incorrectly reported and this may lead to content being off-screen. In typical Apple-style they launch a device with a kludge and expect everyone to do what they want.
I suspect some kind of tweak to IOS Safari will allow users to toggle screen size to include the notch or not.