This isn't to be confused with Google Photos though, the free storage solution, which will still be active. Google will release a tool to easily transition from Google+ Photos to Google Photos, and if you miss the deadline to do this, your photos will be stored in Google Takeouts.
The reason for the shut down seems to be that Google+ Photos simply became obsolete, especially when they have a good photo solution app with Google Photos (without the +).
Google Photos was launched in May this year.
Check out Google's release on the "+" shut down here.
