Take for example this question here on MSE. It is deleted. Users with less than 10k reputation, or without a diamond, or not the author of the question (who happens to be me) only see a friendly "Page Not Found" message, which is all good.
However, by manually changing the URL to the timeline view of the question, any user can see it. Even with less than 10k reputation, even totally anonymous visitors, e.g.
This is showing details like when the question was posted, when it was deleted, and by whom. If there were edits, it will also expose the full contents of the now-deleted question.
All in all, this should not be enabled, and the timeline should return 404 for those without the privilege to see the post, same way as it used to do before it was re-designed and same way the revisions page does.
This also applies for answers, not only questions.
Just another example - random deleted question on arqade, with tons of details I should not see.