What it is
NoML proposes a simple extension to existing robots-style signaling: a noml value that can be added to HTML meta tags or X-Robots-Tag headers. The goal is to let publishers keep content discoverable in search while separately asking that it not be used for machine learning.
It is intentionally lightweight and easy to deploy, but it remains an advocacy proposal rather than a settled web standard.
Watchouts
NoML is a proposal rather than a broadly adopted standard, and it depends on voluntary compliance by crawlers, search engines, and downstream API users.