There must be some loyalist death guard chapter that still exists in 40K. But we just don't know it. Is that possible?!
The Council of Terra kept samples of all the gene-seed from all the Legions. Following the Heresy and the subsequent Foundings there are heavy hints that some Chapters were created from traitor Legion gene-seed. Two of the bigger hints are the Silver Skulls from the Iron Warriors and the Minotaurs from Emperors' Children.
We also know several traitor companies stayed loyal (Dantioch of the Iron Warriors being the more famous example). In a galaxy where several companies from the parent Legion are spread out, there is no doubt that some traitor companies stayed loyal and were accepted (with close monitoring) into the Imperium. With record-keeping so poor in the Imperium, it would be easy for these companies to erase their tainted past.
'Let us speak now of the death of kings...' - Richard II
Sons of Horus 131st Company "The Kingslayers" http://aus30k.com/30...itle=L-XVI-3752
'Better to reign in Hell then serve in Heaven!' - John Milton, Paradise Lost
'What is it about my leadership that makes you think I tolerate failure?' - Ezekyle Abaddon, Black Legion