I've upgraded an iPad app to support iOS10.
The app is used to connect to a MFI supported bluetooth device.
Since the upgrade,
[[EAAccessoryManager sharedAccessoryManager] connectedAccessories]
Ok, I've found what changed.
Starting with iOS10, the plist.info field
UISupportedExternalAccessoryProtocols is case-sensitive.
I fixed it to exactly match the actual device protocol, and now
connectedAccessories works again.