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These ones....
That's an edge, not a corner. iPhone's currently have rounded edges. They've always had rounded corners.
Anyway, I digress. Flat edges would be really nice to have again on the iPhone. Especially because a lot of people still find the bezels to be very thick (myself included). The rounded edges contribute around 30-40% to perceived bezel thickness.
Here is Apple's own diagram of the iPhone Xs Max:

That light grey outer edge you see in the image is the rounded edge of the iPhone. Flat edges will make the iPhone appear to have significantly less bezel.