They can be crafted, it's just they're rarer (I seem to think there's a chart that all slayer applications consults, and Repond and Undead are weighted to be the rarest).
I miss the days during AoS that opposed slayers didn't cause the user to take double damage - I had a Repond/Undead double slayer broadsword from the Paladin new character quest that I enjoyed the hell out of (back when you didn't have the 3x cost penalty on spamming specials, and even when JOAT counted for special moves in early ML, I'd spam the hell out of AI at T-maps with it).