I tried it: https://news.ycombinator.com/item?id=48410589`/llm.txt

Result: no such item.

From where do you got the idea that adding /llm.txt to urls will produce markdown?

here: https://llmstxt.org/ and obviously it doesn't automatically produce markdown, it's something the website needs to provide (e.g. https://pydantic.dev/llms.txt)