Is doom scrolling rotting our brains?

Other way around. A lot of things are rotting our brains, and doom scrolling is one of the results. There's a positive feedback loop, probably, but we were rotting our brains long before doomscrolling.

Neil Postman's 1985 book "Amusing Ourselves To Death" describes a similar phenomenon with TV and radio. And that was before cable news brought us "doom-viewing" with the first Iraq War and 9/11, where people felt compelled to tune in continuously for updates. Even between crises, people watch news for enjoyment, usually to be told about all the terrible things happening.

In my opinion, yes. I think it desensitizes us to new information, content, and experiences. Humans didn’t evolve side by side with the current way of life. However I don’t know how these researchers could prove it. You can’t feasibly conduct a decades long A/B test of people who use and don’t use smartphones or social media, right? At best you could rely on self reported habits maybe? But that doesn’t seem like enough.