Did the countdown clock start when Google launched two other consumer-focused messaging apps with a mobile focus? Or was it the recently-announced demise of Google+, where Hangouts got its start in 2011 before relaunching as a solo act in 2013? Of course, Google Talk/Gchat hung around until 2016, so there's some precedent for an ending that could come later than anyone expects.
Hangouts has appeared as an afterthought in Google's plans for an Assistant AI-powered future -- with RCS expanding on SMS, Android Messages is bringing texting back, it's almost poetic -- so it's tough to see it going much further, rumor or not. Keeping it around for business cases is the only thing we've heard actual news about for the last couple of years, and the last person to tell us its consumer facing service "isn't going anywhere" works for Facebook now.
Update 12/1/2018 9:20AM: Googler and Hangouts boss Scott Johnson tweeted about the story, calling it shoddy reporting. He claims that no decisions have been made about Hangouts' shutdown time. At the same time, he said existing users will be upgraded to the currently enterprise-focused Hangouts Chat and Hangouts Meet -- which would probably only be necessary if "Hangouts classic" goes away at some point in the next couple of years. In response, 9to5 claimed a second source had backed up its story.
Update 2 12/3/2018 2:34 AM: A Google spokesperson gave Engadget the following statement, saying the company hasn't (publicly) announced when it will close Hangouts, but when it does, you'll be able to use Chat and Meet:
In March 2017, we announced plans to evolve classic Hangouts to focus on two experiences that help bring teams together: Hangouts Chat and Hangouts Meet. Both Chat and Meet are available today for G Suite customers and will be made available for consumer users, too. We have not announced an official timeline for transitioning users from classic Hangouts to Chat and Meet. We are fully committed to supporting classic Hangouts users until everyone is successfully migrated to Chat and Meet.