Update from the Collector for ArcGIS Blog
"Collector for ArcGIS is back in iTunes
We are happy to report that Collector for ArcGIS is back in the App Store and the update issues that we introduced with version 10.3.4 is resolved. The current released version is now 10.3.5 and available in the Apple App Store. Our deepest apologies for those affected by this regression."
Previous Alert Information from Oct 15, 2015
An issue has been identified in the latest version of Collector for iOS (10.3.4) that is available on Apple’s App Store. If you are currently using version 10.3.3, please do not upgrade to version 10.3.4. Also, please turn off automatic download of the app updates on your device. Esri is working on this issue and will have more information soon.
Updates for this issue can be found on the Collector for ArcGIS blog page, or in the Esri Knowledge Base Bug article Collector for ArcGIS (iOS) 10.3.4 Has Been Temporarily Removed From the App Store.