All songs ever purchased will continue to play locally but be unavailable for download as of the 1st of August.
EA stated that the reason for this decision was the ending of it’s license agreement with Harmonix:
EA’s licensing agreement with Harmonix is ending and as a result, EA is discontinuing downloads of Rock Band iOS and Rock Band Reloaded iOS on the App Store after 31t July.
After this date you will only be able to re-install purchases if they are synched to your iCloud backup.
You will be able to restore your songs by iTunes provided you have an iCloud backup. An iCloud backup can be created before or after July 31 by selecting backup to iCloud in iTunes.
The removal of Rock Band from the iOS App store was talked about earlier this year in the form of an in-app warning. It’s deadline was 31st of May, well… it was close enough.