Comment 1 for bug 1476661

That's because the interface is based on what exists in the bib data. Specifically, only terms in use by bibs, or authorized headings /not/ in use where the unauthorized headings /are/ in use, show up in browse.

There may be a way to show unused unauthorized headings with some development, but it would take a little work. You'd need to start by changing metabib.browse_pivot() to use metabib.browse_authority_pivot() instead of metabib.browse_authority_refs_pivot(). In the end, you're going to end up with an entry for every single 4xx, 5xx, and maybe 7xx from your authorities, meaning tons of "dead" browse rows. Maybe worth a try to see if you like the output, though.

Could be made a switch, if it's a reasonable choice for some sites.