Once upon a time, I was able to get most of my communication done through Hangouts. Between its support for standard SMS, Google Voice, and normal Hangouts conversations, it was an incredibly useful tool. It could even merge SMS and Hangouts conversations into the same thread, much like iMessage on the iPhone.
Over the past year or so, Google has been separating all this functionality into standalone apps. Google started asking SMS users to switch to Messages, then removed merged conversations, and finally launched a new separate Google Voice app. The company has also been trying to move people off Hangouts itself, with its new Allo and Duo apps.
Now Google appears to be finally pulling the trigger on Hangouts' SMS integration. In an email sent out to G Suite administrators, Google said that a warning (pictured above) will appear to Hangouts users starting March 27. The email also says that SMS support will be completely removed from Hangouts on May 22.
The message states that Google Voice users will not be affected, but with all the improvements in the standalone Voice app, you might be better off switching anyways. No changes were announced for SMS integration with Project Fi, which allows texts and calls to go through Hangouts, but I'm almost certain that it will stay (at least as long as Voice integration does).