I actually think in practice the meaning has always been "things I dislike". Before AI you could see it applied to all kinds of things in media, WWE is slop, So

I actually think in practice the meaning has always been "things I dislike". Before AI you could see it applied to all kinds of things in media, WWE is slop, Soap Operas are slop, Genre Fiction is slop. It's almost exclusively a pejorative based on taste, intended to throw scorn on what other people enjoy. When a person uses it I stop listening, because essentially the speaker has stopped saying anything of value.