SMS was first moved into Hangouts in 2013 as part of Google’s vision for communications, but had a quick change of heart when it launched a dedicated SMS/MMS app called Messenger (now Android Messages) in 2014. Today was the day SMS dropped off the face of Hangouts.
Google has also launched two other messaging services: Duo for video and audio and Allo for advanced chatting. But it was clear from day negative one(?) that Messenger/Android Messages was the main app Google wants users to migrate to.