Problem/Motivation
Under Block Layout there are three pages. Two of these (Block layout and Block types) are structure and site-building tasks. Any configuration or new block types are exported in the configuration management.
But the Custom block library page is a page to create and edit content. Administrators on a production website need to be able to edit this content - but usually they should not be able to change block layout etc at the same time.
There already is an issue to provide more granular block permissions, or at least a separate permission that only allows to edit an existing block (#1975064: Add more granular block permissions and #2809291: Add "edit block $type" permissions). This would solve part of the problem because users then can't break the site.
However, users still require access to the whole Structure and Configuration part of the site to get to the Custom block library tab on the Block layout page.
Proposed resolution
Move the "Custom block library" as a submenu item and/or tab on the content page.
It would then also be possible to move "Block types" up one level as a tab on "Block layout" page.
Remaining tasks
User interface changes
This is a UI change, and therefore the hook_help page also needs edited to correct the locations and paths.