Going back to the 2 missing Primarchs there is also a subtle hint as to what happened to their Legions in one of the more recent Heresy novels, First Heretic I think.
We know, or rather it is suggested that at least one of the missing Primarchs in some way deviated from the norm and rebelled against or displeased the Emperor in some way.
The Primarchs were erased from history, but what about their Legions of Astartes?
It is hinted that they were absorbed into the ranks of the Ultramarines, which is why the 13th Legion is by far the largest.
Could this be why Guilliman was so adamant they follow his Codex Astartes to the letter, and why they stamp out any free thinking or deviation from Guiiliman's teachings?
Is there possibly going to be a darker side of the Ultramarines revealed?
And is it a coincidence that the Ultramarines are the 13th Legion?
Obviously pure speculation on my part, but they have been hinting a bit towards something along these lines.
Also the original Legiones Astartes were based on the 28 Roman Legions. 3 of these Legions were famously lost without trace whilst on campaign in Germania, and very little information about what happened to them was recorded.
However it was then discovered that all 3 Legions had been massacred, mainly due to the incompetence of the General leading them, and apparently all 3 Legions were erased from Imperial Roman records.
Their remains were never found.
Games Workshop always said that the 2 missing Primarchs were in some way representative of the 3 missing Legions, and as they were never found, then they would never reveal what happened to the 2 missing Primarchs.
However Archeologists are thought to have discovered the remains of one of the missing Legions about a year ago, so you never know.....