Fallen rules were reprinted in
WD 464, almost word for word from Vigilus Ablaze, and the Datasheets are still in the 8th ed.
CSM book, because they never bothered to put them into Vigilus OR
WD 464.
Here's the Goons response to
WD 464:
https://www.goonhammer.com/im-not-mad-just-disappointed-fallen-in-wd464-reviewed/
Now what I will say that the Goons don't: there are some really cool conversions in
WD 464... But they're all conversions that can't actually be fielded because they have no rules- things like Fallen Dreads, Fallen Terminators, etc.
Last but not least, I know the
DA supplement has rules representing the Fallen in the Crusade section, but I don't own the book, so I can't say what they are. I don't know if it allows a unit in an army opposing
DA to be keyworded as Fallen and hunted as an Agenda- that might be a way to tag a unit that isn't in the
WD/ VA specialist detachment as Fallen, but
RAW, the keyword would disappear as soon as the enemy isn't
DA. I also don't know if it confers any bonuses or mechanics to the tagged model.
The Fallen are as disappointing as they've ever been. There is a new
CSM coming soonish, so you might see something made of them, but I wouldn't hold your breath, especially with the announcement of the new anti-subfaction soup rules and the knowledge that Fallen will likely continue to be elites.