Email is communication between two people, and is a complete different matter to if a person wrote in their iOS Notes "Feb 17, I've been fantasising eating BWStearns dog. He looks yum. Should I or should I not?".
And if you dog really got murdered, that note will implicate whoever wrote it, even if they didn't do it! Thinking about performing a criminal act is not a crime. Conspiring to do it is.
But let's say that I/you killed your/my dog is not in question. The killer's notes on dog killing are relevant to the case. Your iOS notes are no closer to being part of your brain as a dead tree journal is. Other than (as someone else brought up) a hypothetical brain scanning technology that claims to reveal thoughts[0] I can't think of a category of digital evidence that is so substantively different from analogue predecessors as to warrant folding the whole category into some protected class of information.
[0] I do think that precedent regarding the inadmissibility of the polygraph will probably protect us on that front for at least a while after someone pulls it off.
Oh yeah, sorry. Distinguishing between premeditation or not in order to charge and sentence properly. Without premeditation there may be no crime at all. I did not spell that out well and only touched directly on it in the first post. The point is that it's evidence and not really different than if I wrote it in my dead tree diary. I'm hobbled a bit here because I'm trying to make the very unsexy point that most electronic things are not unique because they're electronic (kinda like how "doing it on a computer" shouldn't be sufficiently novel to justify patent protection). The extent to which "devices becomes more and more an extension of the brain" is at the moment not far enough to justify unique treatment to your iOS notes.
And if you dog really got murdered, that note will implicate whoever wrote it, even if they didn't do it! Thinking about performing a criminal act is not a crime. Conspiring to do it is.