As it happens, there is a mod-only tool called "Orphan Wikis"

https://serverfault.com/admin/orphan-wikis

And in that list I do see one for DFS:

https://serverfault.com/posts/292561/revisions

So yes! Wikis do go missing when tags are merged. About the only thing we can do about that is to take the content of the merged tag and copy it into the merge target; but that means the wiki-credit goes to the moderator doing the merge not the person who made it in the first place.