What does the 'access_civicrm_nav_link' permission allow?
If you do a search at https://github.com/civicrm/civicrm-core and https://github.com/civicrm/civicrm-wordpress there's a note about it being changed and the lack of search results indicate it's no longer used.

EDIT: At github you can go back as far as v4.4 and see that it's not used, and while the old svn repo is no longer available you can see from https://issues.civicrm.org/jira/browse/CRM-12731 that it was used at that point and possibly even removed as part of that issue, somewhere around 4.2.