And "That’s so slop" sounds awkward. But mostly I think the kids haven’t even heard of "AI slop" which isn’t really in the general lexicon, at least yet. That is, the prevalence of its use in these circles is a bubble effect. I doubt anybody ever uses it in kids' Tiktoks or Youtubes or whatever.
So you're telling me that a word which has "slop" in its origins is itself the origin of "slop"?
This must be the first case of circular etymology!
You might object that those are really back-and-forth, but so is the claim being made here about slop. (I kind of doubt the claim though.)
I don't think your examples apply though. The dog example is just dog meaning sausage, sometimes with a temperature quality added (or ignored), but always a sausage with no other connotations.
The pepper one, maybe a little, though according to Etymonline [0] pep is short for pepper, so it's all the same really.
I think our tendency to attribute "novel" word usage to 4chan might reflect more on us than on our language.
I'm a little short on primary reference material, but here's an example from 1917: https://www.gutenberg.org/cache/epub/22692/pg22692-images.ht.... Conversation on language log has at least two others from 1897 and 1920 respectively.
“Slopping the hogs” is probably where the modern negative interpretation comes from of forcing garbage onto someone.