Update: Slides is not available on iOS but is currently available for Android.
After having acquired Quickoffice, Google had made ported many of the document suite's features to its Google Drive service. As such, Google will be retiring Quickoffice and migrating users who need native document editing and creation to Drive's standalone Docs, Sheets, and Slides apps. The move will affect both iOS and Android users as Google suggests that users migrate to those apps instead.
"With the integration of Quickoffice into the Google Docs, Sheets and Slides apps, the Quickoffice app will be unpublished from Google Play and the App Store in the coming weeks," Google said. "Existing users with the app can continue to use it, but no features will be added and new users will not be able to install the app."
Users can download Docs, Sheets, and Slides for free now from either the App Store or the Google Play store.