Google is said to replace Hangouts with Duo on Android's default suite of apps.
Android OEM's are required to install a bunch of apps as part of Google Mobile Services (GMS) package. Till now this included Hangouts as well, which is company is now offloading to the enterprise tier. According to the message received by GMS partners, come December 1st the core GMS package won't come bundled with Hangouts for new devices, instead they would be required to bundle Google Duo.
Now, this doesn't mean that Google is killing Hangouts, you will still be able to download it from the Play Store.
Here's an excerpt from the alleged email:
Today, we are announcing that Google Duo will replace Hangouts within the suite of core GMS apps, and Hangouts will become GMS Optional for telephony products. This change will take effect on December 1, 2016.
Given the simplified interface of their cross-platform Google Duo app, the company is aiming to make it more ubiquitous for Android users. But even with similar exposure, Hangouts never really reached mainstream users who stuck with Facebook Messenger and WhatsApp.
Adios Hangouts, you won't be missed!
Source: Android Police