llms.txt is a proposed plain-text file placed at a website's root that aims to tell AI systems which pages matter most, similar in spirit to robots.txt. Adoption exists, but current evidence shows little measurable effect on whether AI engines actually cite a site.
Large-scale analyses, including studies by Ahrefs and SE Ranking, have found minimal real-world impact so far. It is cheap to implement as insurance, but it is not a priority lever for AI visibility on the current evidence.