Warning slight spoilers for those who want 100% nothing tp do with campaign until game comes out!
If this image is correct (and why wouldn’t it be) chief dies on October 27th 2558 same day Locke goes too Sanghelios!
My theory is that this whole chief is dead thing is just a way of keeping Locke hunting down Chief a secret!
And lol not just any secret top secret cause everyone knows thats the toppest secret!