It's fairly easy to quite thoroughly "de-slop" writing: Just feed chunk by chunk to a an agent that you make compare the writing to a good piece of human writing, and adjust the writing to match. It won't address structural/content issues, but all the major models are perfectly capable of copying the tone and style of a particular style of writing, and in doing so it tends to remove most of the rough edges.

(The corollary is that the LLM writing you notice is mostly going to be from people who aren't actively trying to hide it from you)