'Discourage search engines from indexing...' setting merely sets a book to private about pressbooks HOT 1 OPEN

This dates back to 2013 or so:

The reading options page is not shown to Pressbooks users (you can navigate to it if you know the URL but it isn't linked anywhere in the dashboard navigation). Instead, the blog_public option was repurposed on the Sharing and Privacy options page and used to determine whether the book is public or private (see implementation in the modern McLuhan/book theme here: https://github.com/search?q=repo%3Apressbooks%2Fpressbooks-book%20is_book_public&type=code).

If you want to add another option to toggle search engine visibility, the right approach would probably be to add a new option to each book to control whether the book is public or private, defaulting to value of the blog_public option, and restore the core blog_public option to its original function (which is working as intended; when set to false it adds the following meta tag to all pages of the webbook:

<meta name="robots" content="noindex, nofollow">

(See here for an example: https://integrations.pressbooks.network/sasstest/)

from pressbooks.