Well, what the title says. It struck me just today, when I realised there is this long-running talent missing. It was definitely in DH, RT and DW and I could understand its lack in Black Crusade, but why can´t the Guardsmen armor themself with contempt? Is it a mistake or design omnission?

It does seem like an option that doesn't have a solid reason for not being there. The only thing I can think of is that it wasn't included because a character retiring from corruption shouldn't come up that much. Not to mention this is the imperial guard so a character dying for whatever reason isn't that big of a deal and is easily replaced.

That being said it wouldn't be hard to include as a house rule. Tier 2, willpower/defense, requires 35 wp?

Perhaps the copy/paste gremlin hared off with it? As you say, it wasn't included in Black Crusade, and Only war is an extension of those refined rules. Just as there were copy/paste bugs that made the jump from Rogue Trader to Deathwatch, so too might this have happened here? And only now, when it may be too late to make the addition is the mistake being realized?

Ah, well.