Even with its Allo messaging app launched, Google has said that it has no plans on shuttering Google Hangouts. Still, it has started pre-installing Allo on new Android handsets instead of Hangouts. While Google might not be ready to eliminate Hangouts, it is no longer allowing third party apps to use Hangouts' API. And starting on April 25th, the API will no longer work.
This means that any app that uses Google Hangouts won't be available after that date. Google plans on using Hangouts as a tool for meetings which means that Google will be aiming it at enterprise use.
There are some apps that will still work with Hangouts past the April 25th drop dead date. Those include the the DialPad and RingCentral apps that can be used to dial into a call. Hangouts will also continue to work with enterprise communication tools like Slack, and with Google's own Hangouts on Air broadcasting tools like Toolbox, Control Room and Cameraman.
"At the Google Horizon event in September 2016, we previewed a new experience for Hangouts focused on meetings. In order to streamline our efforts further, we will be retiring the Google+ Hangouts API that enables developers to build apps for the older version of Hangouts video calls. This API was originally intended to support social scenarios for consumer users as part of Google+, whereas Hangouts is now turning to focus on enterprise use cases."-Google