The orphans directive is supplied by the orphans plugin.

This directive generates a list of possibly orphaned pages — pages that no other page links to. Example:

[[!orphans  pages="* and !blog/*"]]

The optional parameter “pages” can be a PageSpec specifying the pages to check for orphans, default is search them all.

Note that it takes backlinks into account, but does not count inlining a page as linking to it, so will generally count many blog-type pages as orphans.