Apple did not include a pressure-sensitive 3D Touch display in the iPhone XR but rather introduced the new ‘Haptic Touch’ feature which is basically a normal long-press gesture with haptic feedback to imitate 3D Touch.
While iPhone XR users can already do things like holding down the spacebar to activate keyboard trackpad mode, or using a long press for the lock screen torch button with Haptic Touch, the ability to view rich notifications is not yet supported.
As pointed out by 9to5Mac, the latest iOS 12.1.1 beta 2 has added the ability to expand notifications with Haptic Touch on the iPhone XR, confirming the feature will be enabled for all users in the next iOS update.
iOS 12.1.1 will allow users to press and hold on a notification banner to see the rich content and action buttons on the iPhone XR.
The only difference between the Haptic Touch and the 3D Touch activation is the amount of time it takes for the system to respond. A 3D Touch device can unambiguously respond instantly once it detects an increase in force, whereas the system must wait to check if the user is long-pressing on the screen or simply pausing during the middle of a different touch action like tapping or scrolling.
It must be kept in mind, however, that some 3D Touch features like Peek and Pop are unlikely to make their way to the iPhone XR because they rely on distinguishing different levels of pressure, which is not possible to imitate with a long press.