I did a story once where the the action gradually departed the main text and shifted into footnotes on the page.

The whole story was silly, and that particular page was "stunt writing", so I'm not quite sure that it qualifies.

There is also: https://en.wikipedia.org/wiki/The_Manuscript_Found_in_Sarago... which was made into an interesting movie: https://www.imdb.com/title/tt0059643/

That's more about recursion than anything.