[dredmorbius]

A sitemap simply tells you what pages exist, not what's on those pages.

Systems such as lunr.js are closer in spirit to a site-oriented search index, though that's not how they're presently positioned, but instead offer JS-based, client-implemented site search for otherwise static websites.

https://lunrjs.com

[TylerE]

How would this help anything? It would make the blackhat SEO even easier, if anything.

[dredmorbius]

The results could be audited.

Fail an audit, lose your reputation (ranking).

The basic principle of auditing is to randomly sample results. BlackHat SEO tends to rely on volume in ways that would be very difficult to hide from even modest sampling sizes.