For example, this question's timeline is 404 for 10k users, who are supposed to be able to see deleted posts.
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This just kinda happened when we released the new timeline. Kind of. What happened is we didn't have any permissions on it at all when it was initially released. That's been corrected so it only shows to 10k users.
Enjoy seeing your timelines.
Given that 10k users can see the revisions list of deleted posts, I think it's reasonable to be able to see the timeline view of a deleted question.
Part of the issue right now is that the full history is not available in the timeline view. For example, if you look at this post here: revisions timeline, you can see that events related to deletion/undeletion do not appear in the timeline (close/reopen events are shown -- example with both a close and reopen).
Jeff has said that he plans to have the timeline view publicly available at some point. I assume the dev team is still working on this, although I'm not sure why the entire page is blocked when the view of a deleted question is requested.