So from reading the CBPeripheralDelegate documentation, it appears that RSSI and peripheralDidUpdateRSSI:error: were deprecated with iOS 8.
Xcode warning:
'rssi' was deprecated in iOS 8.0
I noticed that my signal strength indicator was no longer being updated, so I did some research and found a new method ([CBPeripheralDelegate peripheral:didReadRSSI:error:]) that supposedly get called back asynchronously after calling the readRSSI method. Unfortunately, this method does not seem to get called back either, even though I did set the parent class as the CBPeripheral delegate. 
Has anyone else had issues with the iOS 8 CoreBluetooth updates?
There is a known issue that has been posted about this on the Apple Developer forums to which I have an open radar as well.
The issue seems to be that the new callback method peripheral:didReadRSSI:error: is never called on iOS 8 specifically after the initial connection is made to a peripheral. The only resolution I have found is to reset bluetooth on your phone by turning it on/off or restarting the phone. 
Here is the link to open radar I filed.
Just tested my example that was displaying this issue with the current 8.2 release and I am no longer seeing any issues with the new iOS 8 delegate method never being called. I am marking my radar as resolved per the 8.2 release.
In iOS 8.1.1, peripheral:didReadRSSI:error: will only get called for peripherals that you are connected to. If you've merely discovered the peripherals with a scan and then called readRSSI on them, the callback will not be called.
Try connecting to the peripheral first (via CBCentralManager's connectPeripheral:options:), and then calling readRSSI.
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