There was some speculation a couple of weeks ago when a leaked image from the new
40k rulebook was revealed, suggesting at first glance that the Emperor was dead. See
http://natfka.blogspot.co.uk/2017/06/leaked-image-master-of-mankind-dead.html
However, the full paragraph can be read in the eBook edition just be selecting the text!
It actually says:
"The Custodes, for long millennia the bodyguard of the Emperor, were charged with the unthinkable – to leave his side and strike against those who would see the Master of Mankind dead."