read.amazon.com/kindle-notebook is now redirecting to Scribe FAQ (question 5. mentions only iOS and Android apps, I don't know if it ever referenced the web page).
The Notebook web viewer has been going for months. Hopefully they will bring it back. It had stopped working with Safari a few weeks ago, maybe this is their way of addressing that while they figure out how to fix it (Firefox, Edge and I assume Chrome didn't have issues).
I had just been working on a script to get a list of notebooks and their folders and guid's from the web page. I guess that little project is on hold for now.