Actually he had gotten rejuvenat treatments throughout his career, because he was a well-known HERO OF THE IMPERIUM and of course very useful to Amberley Vail (in many positions ^^).
But, yeah, as of Cain't Last Stand he was getting on in years. But, although he was interred with the fullest measure of military honors, he still is kept as alive on the records of the Munitorum because he turned up so many times alive after being declared dead.
That being said, I am unsure how much the timeline was advanced with 8th edition. The Cain archive was published in M42 by Amberley Vail (who apparently did survive a few hundred to a thousand years, so make your own conclusions there) and the article on Cain at Lexicanum does not give a firm date on his death, stating he died "in the first or second century of M41".
So, if the timeline has not advanced, say, five hundred years or so, he still could still turn up somewhere, with faithful Jurgen at his side.
This is, btw, the only expertise I got on WH40K, since I own all the Ciaphas Cain novels, but nothing else of the setting. Too grimdark normally.
