The PDF Release Notes go back to release R2012b. The document is 666 pages long. I suspect that a combination of size and frequency of use was behind the decision to limit how far back the HTML Release Notes go.
One of the changes that was part of the major graphics system update in release R2014b was that colorbars are no longer axes objects, they are their own type of object. See page 15-20 (aka page 576) in the PDF.