I can go back in browser history and load one of my notebooks, so the site is still there, but the front door is closed.
If I replace the guid in the URL with the guid of the notebook, then I can view any notebook (assuming browser is already logged in to my Amazon account):
https://read.amazon.com/render/notebook?notebookId=[notebook-guid]&language=en-US&theme=Light
There are folder GUIDs as well, but when I tried modifying the URL to use one that I'd captured previously with my script it would not open. I might not have guessed the correct URL format (tried '.../folder?folderId=...').