When Google AdSense detects that a thread has violated it's policy, it would send an email with a direct link to a thread and ask to remove it in 72 hours. I had these several times with vBulletin, and I just soft deleted them with no problem and Google left me alone.
However, this is not the case with XF. When I soft delete a thread, and a gust (Google bot) would try to visit it, it would get a username/password request (because admins/moderators still have the ability to visit this thread). Google thinks that the thread is still there, but is accessible to users only, and therefore keeps sending notices that this thread is still exists and wasn't removed. Only when I hard-deleted the thread, the issue was resolved.
In vBulletin, when a thread was soft deleted, a user would get a soft 404 ("you don't have sufficient permissions to access this thread/section"), so Google would think the thread is gone.
Thus, I think that a soft 404 would be more appropriate for soft-deleted threads in order to keep Google on track.