The moderators aren't responsible for filtering shitposts.

They're removing child abuse and violence (including rape, beheading videos, etc), in addition to racist, *-phobic, etc const. I'm guessing the trauma is more due to the former than the latter, but on the other hand there's a different between seeing the occasional bigoted comment and seeing them solidly for 8 hours a day, every day.

Based on what content I see getting taken down I have a hard time believing this.

Any data available showing the type of content and the frequency at which it’s seen?

I handled abuse reports for a ISP in the late 90s and the majority of it was shock/offensive stuff like lemonparty. The real severe things were few and far between even then.

I don't think shitpost are what are causing these moderators to experience PTSD.