Apple: yeah, the iPhone 8 could do Animoji, but they would be lousy Animoji

The iPhone 8 and 8 Plus, it turns out, which also have the A11 Bionic processor and ARKit support, could have in theory been enabled for Animoji, but the resulting facial expression tracking would be nowhere near the level achieved with the depth mapping available on the iPhone X. That is why Apple isn't offering Animoji on the iPhone 8, as this is their flagship demo of what the inimitable TrueDepth camera set is capable of. 

You can cover the IR camera of the iPhone X, for instance, and see your Animoji stop working in low light for an idea what would the experience be on an iPhone 8, not to mention the depth-mapping algorithms that can transliterate every nook and cranny of your face into a talking fox. The moral of Apple's reply seems to be that, yes, Animoji could work only with a selfie camera, but the level of unsurpassed mimicking detail and consistent performance in all lighting conditions wouldn't be up to Apple's standards, hence you only have them enabled on the iPhone X.