locarno24 wrote:
Pretty much.

The Horus Heresy was 'a thing' that the average educated citizen knows existed - you often see references in guard novels to "run like Horus himself was after me" or similar words.

Certainly its existence was known, but as Psienesis says, the details are largely obscured by ten millennia of myth and preaching by Ecclesiarchs who weren't all that sure on the details themselves to start with.

The Primarchs are definitely a 'known thing' - the Loyalist Primarchs essentially sit alongside the highest tier of saints in Imperial Theology. The traitor primarchs..... it's unlikely anyone who hasn't explicitely tried to study imperial legends and theology would know names aside from Horus, as they aren't really published.

Your average pleb would know that the Emperor created the primarchs, and that they, in turn (led by him) conquered the Imperium. They'll know that one of them was Horus, who was a black traitor heretic that they boo at at Mystery plays (one thing mentioned in Ciaphas Caine and Shiria Calpurnia novels both - it's a genuine medieval religious tradition that can best be described as a bible story version of a 'Christmas pantomime'), and that he turned against the Emperor but was defeated.

One of the religious holidays that's apparently common across the Imperium (mentioned as long ago as 2nd edition!) is the Sanguinalia, which commemorates the Angel's Sacrifice - so at least a distorted version of the circumstances under which that sacrifice occurred must be told.

The big no-no that average people won't learn is that any traitors survived the heresy. It's perfectly acceptable to say that there are worlds out there populated by traitors and heretics (in fact the 'siege mentality' the Imperium fosters kind of demands it), but the Imperium doesn't like to let average citizens know that any of them might still be the original traitors from the horus heresy.

Traitor marines are a really bad thing from an Ecclesiarchal perspective - since the Astartes are portrayed as righteous angels, the idea of traitor angels is very destructive... and the idea that the Traitor Primarchs might still exist is worse.

It's like the Imperium's official stance on daemons - The Imperium doesn't deny daemons exist on a vague, theological basis (the warp is dangerous, pray to the God-Emperor, report psykers, pay your tithes, thank you come again) - indeed "from the beguiling of daemons, our Emperor protect us" is a line in the Fede Imperialis, supposedly the single most common Imperial prayer, or that misguided heretics worship them instead of the Emperor (they're misguided heretics, after all) but does not admit that daemons exist in a very real "your husband/father/son died fighting daemons" sense - because suggesting that they can actually manifest in reality, and can corrupt even non-psykers is bad, but worse it's a short leap from there to figuring out the fact that the gods and demigods talked about by the assorted heretics are real, and quite capable of granting you anything you want up to and including immortality (hence the well-known 'no witnesses' approach to Astra Militarum regiments deployed to warzones to support the Grey Knights).

That's one thing to make clear in the whole 'the Imperium kills you if you know chaos exists' or similar phrases. In the average warzone, the Astra Militarum will have guardsmen fighting something that looks a lot like the Siege of Vraks militia, or cultists. Seeing (or even knowing of the existence of) chaos marines and daemons is the rare thing.

The average guardsman - even one who sees a lot of font-line combat - is only ever likely to fight other humans with autoguns or lasguns, ork boyz, and maybe gaunts (depending on where he's based). Which is why Flak Armour is considered a worthwhile investment - because against 'normal' bullets, lasbolts and blades, it actually provides meaningful protection. It's just that stuff without AP5 is a lot rarer on the tabletop than it is in the universe.