Technically, You can no longer make rounded corners for iOS in Sketch. That’s where Egmont Sketch plugin by Interface.Market steps in to save the day.
Apple for the first time started using squircle shapes in icons. Now you can see this type of shape in many UI elements and this is the main reason why you must use squircle instead regular rounded rectangle in app design for iOS. It’s also why many of the rounded cornered elements in Apple’s official Sketch templates don’t scale in a way that allows you to modify corner Radius using native Sketch controls. Egmont helps you create squircle shapes and you can control the rounding on all corners.