Albeit, the Android Open Source Project is distinct from this, enabling the Google services means agreeing to a license with Google that your device will include its bundle of apps.
The company is dropping Hangouts from the list of apps that must be bundled on upcoming Android devices, with Duo to replace it within the suite of core GMS apps, while Hangouts become GMS Optional for telephony products, according to Android Police.
What it actually means is that OEMs may opt to stop including it on smartphones released in 2017, so users would need to download it from the Play Store.
The change to the core GMS package (AKA gapps) for telephony-enabled Android devices will take effect from December 1, and Google will no longer require Hangouts to be bundled with new hardware.
Ironically, Google feels video calling is pretty important to give a push at the moment, something that's limited on most third-party apps, thus will be giving Duo a real chance of success.