Can we ask questions about historical individuals?
It really depends on the question you ask. Questions such as "wasn't _____ the best?" will be closed (should be closed, anyways). The question "Was John Nash a worse mathematician after..." can't be answered objectively. Why? Because definitions of "better" and "worse" differ by individuals. Perhaps you think he was a better mathematician because he was more relatable, while I think he was a worse mathematician because his work was at a lower theoretical level. Because of this lack of consensus, I would view these questions as inappropriate for SE.
Your first question in the linked post: "What do we know about mathematical ability pre and post psychosis, in schizophrenia?" seems much more answerable to me. Why? Because the terms psychosis and schizophrenia have a standard definition, unlike "worse" in the above context.
Long story short: yes, you can ask questions about historical figures if they align with community guidelines (I can't think of a good example right now that doesn't involve some kind of speculation). But no, you should not ask questions that are primarily opinion based (about historical figures, or anyone/anything else).