Spaces, was an experimental group messaging app launched by Google in May last year. Google has now announced it will shut down its experimental app on April 17. But before it shuts down completely, it will go on read-only mode since March 3rd.
The reason is evident that Google is shifting its focus from the not-so-attractive feature towards the most used ones, and they are trying out different ideas egging on the social media market, which is the "in" thing right now.
Previously we have seen Google Talk giving way to Google Hangouts and the news right now is that, even Hangout is going to be replaced by Duo in the coming days.
So, it can be well expected that, Spaces could be replaced by some newly launched app from Google, as the chances seem to be bleak that the "spaces" would go vacant, and there would be nothing to replace that space.