Section 5.2.3.16 (Notation Data) has this:
First octet: 0x80 = human-readable. This note value is text, a note from one person to another, and need not have meaning to software.
To my reading, this says more or less, "this is a note from one person to another except when it isn't". Especially given such notations as preferred-email-encoding@xxxxxxx which are always human-readable, I suggest this:
First octet: 0x80 = human-readable. This note value is text.
It's just simpler.