Schema.org metadata (using microdata, RDFa or JSON-LD) is quite common actually, search engines rely on it for "rich" SERP features. With LLMs being able to sanity-check the metadata for basic consistency with the page contents, SEO spam will ultimately be at a disadvantage. It just becomes easier and cheaper to penalize/ignore spam while still rewarding sites that include accurate data.

The schema.org vocab is being actively maintained, the latest major version came out last March w/ the latest minor release in September.