Short answer: mostly no. A study of 300,000 domains found ~10% adoption, and across 500M+ AI bot visits, crawlers fetched /llms.txt almost never. Google publicly said it doesn't support it.
It's cheap to add for AI agents and IDE tools, but it is not the AI-search ranking lever it's marketed as. Don't spend real effort here.
Optimize the actual HTML AI models read: clear entity definitions, FAQ content, comparison pages, citable facts. That's what gets you into AI answers. Check your AI visibility free.