Update: Apple has provided iMore with the following statement:
"The kernel cache doesn't contain any user info," an Apple spokesperson told iMore, "and by unencrypting it we're able to optimize the operating system's performance without compromising security."
The kernel is the heart of an operating system and handles all the core operations of the computer. In iOS 10, the kernel cache is unencrypted. That's a marked difference from previous versions of Apple's iPhone and iPad software, and has led to some speculation that Apple either changed its policy on security, or that someone screwed up. My understanding is that the reason was something else entirely:
Streamlining the operating system.
Since it contains only the kernel, device drivers, and configuration files — and absolutely no user data — the iOS 10 kernel cache can be left unencrypted without any concerns over security or privacy.
And since it can be, it was.