This question asks about the use* of content which was deleted from StackExchange sites. I wondered, what if the content was deleted because it violated somebody else's copyright, and that copyright holder did not wish to release it under cc-by-sa? If the user who posted the content later realizes their error and self-deletes it for that reason (which is everything they can do to correct the issue), or if deletion is a result of community self-policing (even though, per Thomas Owens' experience-based comment, mods aren't required to act on this) perhaps even before the copyright holder issues a formal DMCA notice (which SE staff handle and might lead to infringing content being totally removed), there might not be a DMCA process, and if the copyright holder doesn't have 10k+ rep they have no chance to see the violation to prompt making a DMCA request.
In my view, this question should be part of Should I be using content from deleted posts on the Stack Exchange network? but the author commented that "The issue of self-deleting improperly used copyrighted material is beyond the scope of this question" and edited the question (original, post-merge-conflict) to set that specifically out of scope. Hence, the present question, which is all about that issue.
Should I be using content from deleted posts on the Stack Exchange network, even if I don't know why it was deleted, and it is possible that the content was deleted for copyright reasons?
*: "Use the content" here means "copy, redistribute, adapt, remix, transform, build upon..." and excludes the "use" that is simply using the knowledge gained from the words of a post to help solve a problem.