Accessibility into android7 there are news?
You should think of "Talkback" as its own app, like any other app in Android. So the version of the Talkback-app usually has not much to do with the Android version. You can have a new version of Android, but an old version of Talkback and vice versa.

Most of the time the Talkback-version in a newer Android is also newer, but you could manually upgrade Talkback.

Some manufacturers like Samsung or Huawai implement their own "TTS - Text to speech engine", which basically is the Talkback-App. For example: The way the TTS reads numbers or weekdays might be quite wrong, the focus-frame might be blue instead of Googles green frame, etc.

From my experience the not-Google TTS-engines are worse than the one from Google, but in the Talkback-Settings you can always switch to the standard Google TTS. 

But to answer your question about Android 5 and 7: Sadly I could not find any change logs that exactly describe what has changed. The best link is this: https://nfb.org/blog/vonb-blog/android-60-and-talkback-45-accessibility-improvements

From my experience I can say that the accessibility usually improves from version to version, especially on WebViews and how things are put into context ("5:00" could be read as "5 - pause - zero zero" or as "5 o'clock"). So if you can use a newer version of Android / Talkback, you will benefit from it.