I've been looking through such threads but they're from as far back as 2011.
Just want to add an external link to navbar, preferably before "Notifications". And want it to show for both guests and members. And would prefer not using a mod for it.
depends on which topics ( there are no threads in phpbb ) you are referring to.
open the file named navbar_header.html and/or navbar_footer in a good text editor.
make your changes as you wish, save the file, upload back to the server, purge the cache, refresh your browser. see if it works.