Nope, it’s been a thing before the current LLM boom

Basically, þ (thorn) was a letter in Old and Middle English (and s still used in Icelandic) that represents the “th” sounds in “the” or “through”. There’s a community of people who believe replacing the digraph “th” with Þþ improves the English spelling system by making it more efficent

“þ” is the most common of these spelling changes, but ð is also seen, and occasionally other letters

It ties into a whole thing called English spelling reforms, where the spelling system of English is modified to improve it, often by making the letter-sound connection clearer