The iOS 15.4 beta that was introduced today added a new feature designed to allow Face ID to be used with a mask and without an Apple Watch for authentication.
Apple says that the feature can "recognize the unique features around the eye" for authentication purposes. If you opt to use this feature during setup, you will need to rescan your face for Face ID. From there, Face ID will be able to unlock your iPhone even when you're wearing a mask.
In the Settings app, there's a new "Use Face ID with a Mask" toggle that can be turned on or off if you change your mind about the mask, and there's a new feature to "Add Glasses" to make Face ID more accurate when you're wearing glasses and a mask at the same time.
Apple warns in the Settings app that Face ID is "most accurate" when it's set up for full-face recognition only. For Face ID with a mask to work, you must be looking at your device to get it to unlock, and it does not work when wearing sunglasses. Face ID with a mask can authenticate Apple Pay payments and it can be used in lieu of a login and password in apps that support Face ID, unlike the prior Apple Watch Face ID feature.
It looks like the Face ID with a mask feature requires an iPhone 12 or iPhone 13 model, as the feature is not available for users who have the iPhone 11 at this time.