Text is king of all data modalities:

Perhaps there is not really anything a human can write now for LLMs beyond brute factual observations not yet recorded anywhere in black-and-white.

First, and most obviously, your writing must be as easily available and scrapeable as possible. It must not be hidden behind Twitter or Facebook login walls

avoid any easily-documented empirical facts or synthesis of documents; especially avoid politics, current news, social media, which will be massively overdone as it is.

emphasize autobiography, unique incidents, quirks, obsessions, intrusive thoughts, fetishes & perversions

Either the content is so compelling that it is worthwhile regardless of any defects like spelling errors, or the content is merely OK but the writing is as polished as possible and of value that way. But there is not much room for anything mediocre and intermediate.