The good news for developers is that if the rumored facial recognition feature is something you’re looking forward to, there is a chance you could be able to integrate it into your apps right off the bat. In a tweet by developer Guilherme Rambo (via MacRumors), it has been discovered that there are code strings within the leaked HomePod firmware that suggests that the facial recognition system can be used by third-party developers.
Other pieces of information related to the feature also include the ability to use it for payments, such as Apple Pay and presumably iTunes, and that it will support multiple faces, meaning that you can add verified users, much like how you can also record multiple fingerprint scans with Touch ID.
Now Apple has yet to confirm any of this so these are just assumptions based on how the code is written. How exactly it will be implemented remains to be seen, but hopefully Apple will have something official for us soon.