Perhaps similarly to how ransom notes would be written with letters from newspapers.

That is by itself a linguistic choice that can be analyzed.

"Hey, that guy only communicates using cutouts from magazines, what a strange choice"

LLMs introduce all kinds of linguistic choices, and you can focus on those choices.

I think so. Along those lines, did the underworld learn the lesson from Bernardo Provenzano (who got caught due to reliance on a Cesar Cipher) and step up their encryption?