I think that's a huge source of disconnect with Gen Z. I don't think they ever had a concept of dark humor.
I mean, I dunno; I think you're right in the sense that Gen. Z doesn't do edgy humor nearly as much but they are still known for irony poisoning and I think you get similar outcomes, just (now) with the veneer of there being, really, a truth to what you're saying.
"Helen Keller didn't really do all that" is squarely a Gen. Z "joke" with just as awful implications as dark humor.