The Sunrise team revealed their shutdown plans for the app in a blog post:

The entire Sunrise team is now working side-by-side with the Outlook team and it's a thrilling moment for us to work on an app of this scale. Unfortunately, as all good stories go, there's a sad bit to it: we're not able to support and update Sunrise anymore. No new features. No bug fixes. For us, that's the definition of a lousy app and it's not a user experience we want to leave you with. For this reason, we'll be removing Sunrise from the app stores in the next few days. On August 31st, we'll officially shut down the app and it will stop working all together.