Well we don't know, but we can assume that they were purged as were the Traitor Legion homeworlds in an attempt to remove knowledge of the Heresy/Traitor Legions. After the Emperor's ascension the Imperium became increasingly paranoid about Chaos' influence (considering what it had done to the Imperium/Galaxy during the Heresy) and I would assume that the Inquisition at somepoint pressed for Isstvans and Davins destruction to remove any possible lingering Chaos taint.
Heresy's Background FAQ. (Fluff Project)
I do love how we don't even need CotE to comment anymore, chances are he has already said something intelligent before that can be re-applied to the current situation.
CotE Reviews: Prospero Burns (HH Review)
, Age of Darkness (HH Review)