There's two dimensions here, one is US-American readers, the other is how a lot of the rest of the (non-English) world is mostly exposed to US culture through (social) media.

But that's more of a thing for millennials, I would've thought younger generations get exposed to more diverse cultures / languages / etc.

Anyway, for British-English full of cultural references, watch some of these compilations https://www.youtube.com/@OneGazillionEccentricGoldfish, Scouse is nearly incomprehensible (to my ESL ears). For difficult US-English full of cultural references, watch The Wire or Treme. Try both without subtitles.

The funny thing is for younger British people this tends to be highly asymmetric - we can (sort of!) understand Scouse or Glaswegian due to growing up here, but also almost everyone under the age of 50 grew up on a steady diet of American TV shows, hip hop etc.

I can understand The Wire fine without subtitles because most of the actors just speak relatively generic African American English instead of a proper Baltimore dialect, and that's no problem at all for someone who spent their formative years consuming Nas and Biggie and all the rest of it.

On the other hand Snoop who is the only main character with an actual Baltimore street accent is pretty much unintelligible to me, but I suspect she would be for a lot of middle class americans as well