We've found it useful just for our own development process. The important part is having documentation (or marketing pages) with an .md extension + "Copy as Markdown" links on pages. Then during development or research your own coding agents can fetch them easier. It also makes you think about how agents consume your HTML content.

The llms.txt part acting as an index is less useful, other than maybe helping plan content gaps. I doubt any of our customers or chat models are using it though.