Most Hangouts video call apps will be retired April 25th, excluding HOA broadcasting tools and enterprise communications tools

There are a few apps which will continue to run past the April 25th date. These include the ability to dial into a call (served by the DialPad and RingCentral apps), integrations with other enterprise communications tools (e.g. Slack), and our Hangouts on Air broadcasting tools (Toolbox, Control Room, Cameraman).

Announcement:
https://developers.google.com/+/hangouts/support-faq

Edited to add: also see this update from +Hangout Toolbox​:
https://plus.google.com/+Hangouttoolbox/posts/3A1rjxL2r3q

Why the change?

This [Google+ Hangouts] 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.

Currently the old Hangout interface that supports apps is only available through Google Calendar event video call links, and Hangouts on Air.

The "new" more-reliable Hangouts video call interface, introduced in 2015, has never supported the apps, so if that is what you've been using you will see no change. 
Photo
Shared publiclyView activity