One of the most significant changes is not even mentioned in the update notes: NCX navigation!
The GoTo button now brings up a list of the navpoint labels, scrolled to indicate your position in the book (current location shown with bold label). It shows navpoint hierarchy and you can collapse and scroll and jump to new location just as with ePub reading systems.
This has always been a weak point of Kindle relative to ePub systems. Not any more.
If the book has no NCX, then you can go to the linked TOC as before (if there's one of those).