iOS 15 Safari brings a new layout where the tab bar containing the URL, is positioned at the bottom as the new default position.
The new layout fills the screen portion above the top of the page with the page background colour, and when scrolling a page, this is also used to fill the top and bottom of the screen. As the page colour is often white, it may be worth considering how your site looks on these devices.
Setting the page BG to a contrasting colour is an easy way to benefit from using this new behaviour to frame the view on iOS 15 notched devices.
Here is a screenshot with the page BG set to white.
Here is the same view with the page BG set to red.
Note how the red background frames the page content when scrolling. IMHO this looks much better than a white area top and bottom.
PS I used a contrasting colour because the top of my pages uses an image, but if it was a solid colour, by matching the page BG to that colour, you can create what looks like the top of your page extending under the notch.