If someone makes a post on any site on the Stack Exchange Network, they grant Stack Exchange a license to (among other things) display and store the work. This license is irrevocable, even if the content is removed by the author. Stack Exchange has the work and can use* it under Creative Commons Attribute Share-Alike 3.0 Unported.
Every person who receives this content is also granted the same license. That means, when I view a post, I receive a copy under the CC BY-SA license. If I wish to use* it, I must do so under those license terms. This post makes it pretty clear that the license is granted to people who see the content.
If I make a post on Stack Exchange and then choose to delete it myself, Stack Exchange still has a license to use* it under CC BY-SA. That means that Stack Exchange can choose to undelete the post.
Stack Exchange has made a technical decision that deleted content is only visible to users with at least 10k reputation points, the site moderators, and some employees.
In this particular instance, the post was deleted and remains deleted forever. No one viewed the post prior to deletion, so no one other than Stack Exchange received a license to the content. Stack Exchange is still making a decision to display the content to a subset of individuals (10k users, mods, staff).
If I am a 10k user, a moderator, or a Stack Exchange employee and I see an interesting (but deleted) post, should I use* it under the CC BY-SA license? Note that I can, legally, use* the content as long as I adhere to the terms of the license. This is specifically a question of culture and ethics.
Does it matter if the post was self-deleted by the user, deleted by the community, or deleted by a moderator? If I participated in the deletion, does that have any impact on if I should or should not be using the content?
For the purposes of this discussion, assume that the post does not contain any content that is illegal or improperly used. The author has the right to grant a CC BY-SA to the entire work. For a discussion that does not make that assumption, see here.
Keep in mind that this is purely a Stack Exchange cultural/ethical discussion and not a legal discussion. Stack Exchange has a license to use* the work and they have granted me the work under the same license. There is a question regarding proper attribution for these posts on the Open Source Stack Exchange.
*: By "use the content", I mean "copy, redistribute, adapt, remix, transform, build upon..." and exclude the "use" that is simply using the knowledge gained from the words of a post to help me solve a problem.