Hmmm…I do not know. I agree that there’s a point where you probably shouldn’t have to say it anymore. There’s no reason to write “the late Emperor Constantine” anymore when he’s been dead for almost 1700 years. It also seems like the more famous somebody is, the less you see their name preceded by “the late” once they die. I don’t know the protocol, though.
Is that something you might find in the AP StyleBook? I don’t have one, so I can’t check. That reminds me, I should have an AP StyleBook.