On the 1.5 HYS regarding the new thread tagging functionality Chris said:
Normally tags will be removed when they are no longer used (specifically, when their last usage is removed).
Before 1.5 I was using [Tinhte] XenTag. After upgrading to 1.5 I used [by PiX-house.com] Importer for [Tinhte] XenTag to XenForo 1.5 to import all tags from XenTag to XF's own tagging system. Please note that when I migrated to XenForo I also used XenTag to import all the tags from vBulletin 3
Now in 1.5 I imported like 20,000 tags from XenTag of which around 1,500 have their "Last Used" value set to blank (nothing) and also the "Total" set to "0" (zero). A week has passed and those tags are still on the system.
I have also noticed something strange: let's say I manually go to a soft-deleted thread that had the tag "hello world" (which currently is being listed with a "Last Used" set to blank and "Total" to zero), on that thread I can see the tag being listed and when I click on it I get the error "No results found". I believe that the tag should not be visible when viewing the thread because it has the "Last Used" set to blank and "Total" to zero. Maybe this was a bug in XenTag/vBulletin 3 not properly deleting tags after a thread got soft-deleted.