When a creator posts content to any Stack Exchange site, they grant Stack Exchange a Creative Commons Attribute Share-Alike license to use (and display and share) the content. This is irrevocable - even if you choose to delete your post, or a moderator deletes your post, Stack Exchange still has a license to use it.
Stack Exchange has chosen to allow users with 10k reputation, site moderators, and some staff to view deleted posts. That means that this class of user can, potentially, receive a CC BY-SA license to all content ever posted on a given site and use it, under the CC BY-SA license.
Creative Commons provides the following rules for providing attribution:
If supplied, you must provide the name of the creator and attribution parties, a copyright notice, a license notice, a disclaimer notice, and a link to the material. CC licenses prior to Version 4.0 also require you to provide the title of the material if supplied, and may have other slight differences.
One of the requirements is "a link to the material". Since the content has been deleted, it is no longer accessible to people who do not fit the criteria who can view it. However, the post still does have a URL (should it ever become undeleted, for example).
Would linking to a deleted post (while providing the other required information) be considered proper attribution under the Creative Commons rules? If not, what would proper attribution to a deleted Stack Exchange post look like?
Note that this does not consider the ethics of using a deleted post. I have received the post and have the right to use it. For a cultural/ethical discussion, see this post on Meta Stack Exchange.