Last time I checked, Corax becoming, living shadow is only in-universe rumour and unconfirmed. Possibility to be sure, but's also possible he is in stasis in the Ravenspire, waiting For The Emperor or corpse drifting in The Eye of Terror after Lorgar beat the shit out of him or something like that.Like, it would be interesting if the Raven Guard chapter had unlocked the ability to basically become warp beings similar to their Primarch because he himself unlocked that without loosing his Primarch soul.
For example, here's what "First Founding" background and art book published in 2023, about the titular organisations has to say on fate of Corax.
Conversely, there is an all-but-secret tale, kept on record within the Raven Guard's Librarius, that in fact Corax never left the Ravenspire, let alone Deliverance. It claims that he locked himself away in stasis, awaiting the time when he would be granted the Emperor's forgiveness for failing to contain the treachery that made itself known in the Isstvan System. Among the most bizarre tales is that Corax eventually turned to a being of pure shadow, an evolution from his ability to disguise his presence to all around even when apparently fully exposed.
Other sources, like "Deathwatch-First Founding" just say that Corax after Heresy left the spire for The Eye and never returned. As far as I'm concerned, Raven Guard succesor should get ability to become harder to see or otherwise hide their presence, for example to muffle the sound they and their gear make but nothing beyond that.