I attempted to install iOS 8.4.1 on my AT&T iPhone 6. I got the iOS 8.4.1 firmware from ipsw.me. I tried to enter pwned DFU but when into recovery so first the custom install fails and second it gets stuck in recovery (no way out except for restoring to iOS 9.0.2. Note: I said restore because the update option will fail). And yes, 8.4.1 is still being signed. It's 8.4 is not being signed.