I am removing some Knob and Tube wiring. Can I leave the NEUTRAL wires hanging in the walls?
I think it's very common to leave disconnected knob-and-tube wiring in the walls.  Leaving a note sounds like generally a greate idea, but I didn't understand where you intended to put the note.

I think the most important thing is to label the circuits going out from your distribution panel so it's clear which circuits still have knob-and-tube wiring.  If you know where the active knob-and-tube wiring is (physically), that would be great to document, too.  It will come in handy if you ever consider blown-in insulation.