Description of the need This is a long standing confusion. In Drupal there are multiple issues for this:

The first issue describes it well:

It is somewhat disconcerting to users who have the 'administer book outlines' permission to see the 'outline' tab and 'outline' fieldset on every single node type, regardless of whether that node type has been configured to be allowed in books. I can see a use-case where that might be a desired feature, but i think its probably relatively rare.

Note, that book module seems to be the oddball here. If you disable menus for a content type not even user 1 has ability to add them to a menu from within the content type.

Proposed solution

Create an additional checkbox in the content/book/settings form that when checked allows book outlines for any content type, not just the designated book-types. By default this is checked, so that any current sites that might rely on "seeing outline information on all content types regardless of config settings" won't be broken by this after upgrading.

GitHub Link: 
https://github.com/backdrop/backdrop-issues/issues/3638
GitHub Issue #: 
3638