The iPhone has had a Power Reserve function that holds back a small amount of battery life to allow features like Find My and NFC unlocking to work even when your device has died, and in iOS 18, Apple seems to be improving the feature further for the iPhone 15 models.
As demonstrated on Reddit, when the battery on an iPhone running iOS 18 is exhausted, the phone can continue to show the time in the upper left corner of the device, which is a small but useful upgrade. The battery icon still shows up, and you'll see "iPhone is Findable" while reserve battery is left.
It appears that this feature is limited to the iPhone 15 models. We tested on an iPhone 14 Pro Max and were not ale to get the time to show up, but it did work as expected with an iPhone 15 Pro Max.