Wasn't it "nine traitor, nine nice, two missing" even from the get go?
Also, IIRC in RT weren't Space Marines more like modern Marines, in that you're recruited and trained and can walk or be booted out to become a mercenary?
Basically, all we know is:
- They're not just excommunicato traitorus, but all records are deleted (from the 4th ed rule book).
- The Primarchs swore an involatable oath to never speak of their two other brothers (Horus Heresy: The First Heretic)
- There were statues of them in a great hall on Terra, but they were either removed or destroyed (HH: The Lightning Tower (audio) Sigismund suggests doing the same to the traitor Primarchs, and Malcador makes veiled reference to the others IIRC).
- They MAY (or more likely it was an idea once, but has since been dropped) be Sigmar and Belakor from WFB.
- Inferentially, from everything said about them, they did something so bad the Emperor deleted them from history. It's possible that unlike the Thousand Sons, the Emperor's Children and the Raven Guard their geneseed wasn't recoverable from mutation/corruption or their Primarch was even more of a stubborn cuss then Russ, Vulkan and Khan (who all said "sod off mate" to the Emperor when he turned up).