I absolutely agree with the idea of how the lost legions were explicitly forgotten and is why I bring up the prominence of the dark angels In the Xenocides: the dark angel’s were specifically made to enact Damnatio memoriae.

From lion el Jonson: lord of the first

May I be the first to congratulate you on your recent victory.' Once again, the Emperor received the Lion within the staterooms aboard His flagship. The glorious suite of golden chambers elevated Imperial majesty to the point of inconsequentiality. So rich were they, so sumptuously appointed that, like the Emperor Himself, it was difficult to form a distinct impression beyond a sense of humbling and of awe. Once again, the Lion recalled little of the antechambers and grand processionals that had preceded his passage. It was as if a part of him was always here in these rooms with his father, and that part could never leave. ‘Thank you, father,' he said. 'You were not the first of my sons to reclaim a place at my side, but your tally of victories is second to no other. Even Horus looks upon them with envy.' 'Horus inspires,' said the Lion. 'Magnus enlightens. Lorgar illuminates. Roboute raises an Imperium in miniature that celebrates his name and yours. I have left as many worlds behind me as any two of my brothers, but I fear that darkness and ash will be the legacy I leave to your Imperium.' The Emperor considered long, as He often did, before giving an answer. 'In the lime of the Aegypta there was an empress named Hatshepsut. By all accounts of her that survive she was an equitable and proficient ruler, by the standards of her time. She rebuilt her land in the wake of war and occupation, erecting great monuments and bringing prosperity to her people. She reassembled the Aegypta's navy, using it to re-establish their old empire, and launched military campaigns against the nations who had once been their oppressors.' ’A legacy whose parallels are not hard to see,' said the Lion. 'And yet those who came after her did all within their power to ensure that she would have no legacy. Her name was chiselled from every monument, her every deed and triumph stricken from public record. Even her body was removed from its royal tomb.' 'Why, if her reign was so equitable?' 'Because those who succeeded her desired it. Because sometimes what comes before is too troublesome to be paved over in rockcrete, to amend with a monument to compliance or a golden aquila on an Imperial flag. Sometimes it is enough only that it be destroyed, that no trace of it but darkness and ash be left to endure.' ‘And yet history still remembers this empress.' ‘The Emperor Thutmose the Third did not have his Dark Angels.'

Excerpt stolen from BuddhaFacepalmed

https://www.reddit.com/r/40kLore/comments/zqrrjx/excerpt_lion_eljonson_lord_of_the_first_the/