iOS 15.4 begins rolling out to the iPhone 6s or newer with Face ID improvements for the iPhone 12 and iPhone 13 series
Apple has released iOS 15.4, an update that brings improved Face ID support and the ability to store EU Digital COVID certificates without needing a dedicated app. iOS 15.4 is available to the iPhone 6s or newer, but Apple restricts some enhancements and features to the iPhone 12 and iPhone 13 series.
Source: notebookcheck.net