I would imagine that there would have been some sort of mission by the Imperium to see what they could recover from the battlefield of the Dropsite Massacre. There, if Horus didn't strip the site, would have been thousands of suits of armour, tanks, artillery etc, in various states of repair, and I imagine that they would have been considered very valuable.
Beyond recovering what armour and weapons they can, I imagine that the recoverable remains of the loyalist Astartes, and probably those of the Imperial Army who fought there as well, would have been taken back to Terra or the homeworlds of the Legions and Regiments involved. I can imagine the huge ossuaries built from the bones of the honoured fallen taking pride of place in the fortress monastery of every Legion that lost Astartes in the Isstvan system, and probably each of the second founding Chapters would have a small amount given to them, as sort of a link to their history. We've seen how Imperial forces, especially Marines, like hauling around the bones of the greatest heroes, so I don't think it's too far out there!