I saw this post on MSE, which I thought was pretty funny. Then, I realized the ridiculous amount of downvotes it got within an hour. So, that made me think, what is the most downvoted question in history on MSE or math meta? I am aware of the duplicate question, but it doesn't show any good solutions to find any DELETED posts, and I would like to know what the most downvoted question was of all time, which includes the time before the question was deleted.
It depends on what you mean by most downvoted - whether you mean the lowest score or the most downvotes.
For the lowest score you can simply choose the votes tab and then go to the end of the list. However, you can also create a SEDE query for questions with the lowest score. Using SEDE has the advantage that the data contains also the deleted questions, so you can get a query including the deleted questions.
If you want the number of downvotes (rather than score - which also takes into account the number of upvotes), you can use the Votes table. For example, you can get a query like this. (The queries I have included here can be improved and modified in various ways. And, of course, you can run them on various sites - the links I provided use Mathematics Meta.)