What do they actually see?
- They left the browser window with their question in it open,
- They had a link to the question,
- Or they got at least one comment on the question.
That last is significant, of course. Probably 90%+ of questions en route to deletion will receive at least one comment.
Is there anything proactively pointing them to the fact their post was deleted? A notification in their Inbox? Etc.?
No, nothing, other (again) than any comment notifications.
I posted a low-quality question (with a throwaway sockpuppet) to Stack Overflow as suggested by πάντα ῥεῖ and got it closed and deleted, observing the process as it occurred.
Before the question is closed
- User gets no indication close votes have been cast.
- User does see the question score (aggregate, new users don't get breakdowns).
- Naturally, user sees comments.
Once question is closed but not yet deleted
- User sees the close reason.
- User sees the delete count (which I thought was interesting, given they don't see the close votes).
Once question is deleted
- User gets no notification — nothing in the inbox, no email (not that I'm advocating email!), nothing.
- The question just disappears from their Activity tab, which says Questions (0) and You have not asked any questions.
- If the user clicks through to their list of Questions (but they won't), it again says You have not asked any questions and it has the "deleted recent questions" link.
- If the user clicks the link (but they won't), they see the question listed and can follow it.
- Their Inbox does have at least one entry in it if there were comments on the question.
Unfortunately, what I didn't think to do was to avoid clearing Inbox notifications of comments, and so I don't know if those would still be marked with the red  (or  or ). Presumably they would be.
Of course, if the user had a link to the question, or left it open in their browser when they went away and came back, they'd be able to see it. But if they didn't think to do that, nothing helps them find out their question was poorly-received and deleted except the comment notifications (if they received comments, but I suspect in general they will get at least one comment).