DebateMe - Divinity 2 didn't say this outright, but I assumed that Dragon Slayers learned some innate magic that prevented them from falling to their deaths - a really practical thing for fighting a flying enemy, I imagine.
Actually, you are correct. It's been a while since I played Divinity 2:DKS and my memory is not as clear as I thought. There were no books that stated or implied falling damage or death due to falling.
Where I became confused on this is that in at least one dungeon (I believe more than one dungon, but at least one) in DKS, there is an area where you must make a verticle ascent using platforms and teleporters.
If you make an error, you end up on the ground floor and have to start over. But in these locations you see the bodies of dead people who, I guess, previously tried to make the ascent, but fell to their deaths.
This is a similar inconsistency to mentioning night time in books.
But I guess if Dragon Knights have powers to not fall from death, that perhaps doesn't extend to just any NPC which might explain why there were dead people in those areas.