They're in between the separated blades. I'd direct you to the MMOChampionWoW video entitled "Legion - Artifact Weapons" where they are properly displayed instead of improperly shown as solid colored boxes in the WoWHead viewer.
Every variant of Apocalypse has runes on it.
I'm not arguing here that none of them have runes, I'm just saying that giving a flail to a DK wouldn't be that much of a stretch, even if it has no runes on it. Which it might, who knows. You could add a rune on the hilt, on the ball with spikes, or on the chain if you want to.
But to keep a dead horse beaten (with a flail, why not), Soulreaper variant of Maw of the Damned looks quite runeless.
I mean...
You didn't say none. You did say most though, which is still wrong.
I could only find 2 skins across all the weapons that lacked runes. That's a funny definition of "most".
A flail being a third wouldn't be too much of a stretch no, but it still somehow seems very unfitting for a Death Knight.
I wouldn't mind Unholy getting some kind of huge mace, but some people are upset even at the suggestion of any weapon that doesn't have an edge :P
Giant-Nazgul flail...that's so much better...and i'm sure you can fit at least a couple of runes in there...