The way we do things now, attribute to a site only when the post is claimed by another site, is the correct way to do things. Absent a party willing to claim ownership for the text, it is difficult to know whether the original site/user still wants to be associated with it. We attribute only to the extend that the originating party wants to be associated with the material.
So not only do we not attribute such things to a site, we should not attribute such things until the site requests it. There is nothing to link to, except obscure archives of deleted material, and clearly the material is considered unimportant by others.
Attributing it to the author is necessary, especially if the material has been deleted against the users will. So we should attribute it to a user with the same name, so as to make it clear who the author is, so as not to plagiarize unintentionally, but this is what we already do, and if the user has never logged in here, we should say so. That's what we already do.
I think the site currently has the best possible policy regarding this.