The new $699 iPhone 11 replaces the iPhone XR from last year. While it still makes a few compromises to reach its lower price tag, the device is a major upgrade over the iPhone XR, especially in the camera department.
Apple tends to try and maintain the compatibility of iPhone cases with every new iPhone. However, since the iPhone 11 comes with some pretty major rear design changes, iPhone XR cases are not going to be compatible with it.
While the iPhone XR featured a single 12MP camera at the rear, the iPhone 11 features a dual-camera setup. The Apple logo has also been repositioned and moved to the center of the rear glass. The scenario is similar to the iPhone 11 series which also comes with major rear design changes thereby making iPhone XS cases incompatible with it.
Apple itself is selling official cases for the iPhone 11 and third-party accessory makers have also put the cases on sale or pre-order. So if you plan on ordering the iPhone 11, you need not worry about cases for it since you will have plenty of options on that front as well.