Before Halo CE, 343 guilty spark stays alone on the ring for a long time. Wondering, “Am I the only intelligence left? Waiting here on this ring for a rebirth never truly conceived.” At 13:25: “I have now endured 60,000 years without word from outside the array. I have no way to know whether we saved the galaxy we destroyed.”
So, this shows that Spark did know the rings were activated. I mean, he is the monitor and had to be present on it when it activated. But when the humans show up he recognizes them as Reclaimers saying “We have much to discuss, I have been away far too long, you have been away far too long.” Since he knows the rings were activated, that should indicate to him that these reclaimers have evolved ever since after the activation. Does he assume the new reclaimers have discovered this history already? He treats Chief as if he is already fully aware of how things work, almost activating the ring.
Guilty Spark’s knowledge was deliberately limited as a precaution against his capture by the Flood, so he at least wasn’t consciously aware of the Arks’ existence during his long period of solitude. He also had no way of learning that the Greater Ark was destroyed along with the remnants of Forerunner civilization. Guilty Spark assumed that Reclaimers couldn’t find the Halos until they’d at least discovered their history and purpose.
Bungie’s initial idea was that modern humans were just Forerunners that were re-seeded on Earth after the Halo Array was fired, but as the franchise became popular, that changed as they re-formed the lore into a much tighter and coherent narrative, in which now the Forerunners and humans were descended from the same ancestors, but the Forerunners became quite different because they practiced genetic engineering on themselves to such an extent that they didn’t look human any more.
He knew the Forerunners died based on the cutscene in H2 right before the boss fight with Tartarus. But, that seems like something that was at least a bit of a retcon based on the comment above mine.
Without the Domain there simply was no way of knowing whether or not all Forerunners died. Evidently Spark appears to believe during Renegades that the Librarian might have survived.
Indeed as we know now: some survived. The (Ur-)Didact in his Cryptum, and the party who were on the Lesser Ark and travelled to Maethrillian including Iso-Didact (Bornstellar) and Chant-to-Green among others, who left the Milky Way - potentially for Path Kethona (Large Magellanic Cloud). We do not know their fate.