Releasing iOS 10 GM after the iPhone 7 presentation, the Apple company decided that iOS 9.3.4 is no longer useful to anyone, so it stopped signing SHSHs for this version. Starting this evening, you can no longer install iOS 9.3.4 in your terminals because Apple no longer signs SHSHs for it, so you will only be able to install iOS 9.3.5.
iOS 9.3.5 was released just two weeks ago by Apple, so it was expected that iOS 9.3.4 would no longer be signed at some point. Of course, no one expected this moment to come so quickly, but considering that the release of iOS 10 is only 6 days away, maybe things were meant to be this way.
That being said, iOS 9.3.4 has become history.
