User blog comment:RemosPendragon/Space Marines policy/@comment-2253300-20150917001836

I don't recall having ever discussed Traitor Loyalists when we were making that policy. I'm not sure that I understand the logic behind that particular rule. It's actually fairly established Canon that there were small detachments from the Legions on both sides of the Horus Heresy that chose to fight for the other side. There were loyalists that chose to abandon the Emperor and fight on behalf of Horus, and there were members of the Traitor Legions that fought against the rebels, though notably fewer given how thuroughly the Traitor Legions scoured themselves of loyalist elements prior to the beginning of the Conflict. Now with the "No Loyalist Traitors" rule it makes sense that you wouldn't have any beyond the Great Scouring. As stated the rule was created to avoid a bunch of Special Snowflake Chapters that bore Traitor Gene-seed, a fad of which I was sadly a part of back in the day. Additionally with the Imperium the way it was those detachments of the Traitor Legions that remained loyal had no chance of being welcomed back into the Imperial fold, and if they wren't hunted down they likely would have died off through a lack of support.

Traitor Loyalists surviving on the other hand is plausible. They could easily have survived and hidden from hostiles within the Eye of Terror, and while there would likely be a significant stigma against them amongst the Traitor Legions I couldn't imagine them going out of their way and hunting them down in an organized fashion like the Imperium would do with the Loyalist Traitors. With a Haven and the blessings of the Dark Gods such detachments of Traitor Loyalists could easily have formed Warbands that lasted till current times. It's not like there aren't a fair number of Loyalist Chapters that haven't fallen to Chaos since then. So unless we got a sudden surge of poorly written Warband articles claiming to be Traitor Loyalists that participated in the Horus Heresy I personally see no reason why Traitor Loyalists should be resitricted. Mind you that's just my two cents.