Googlebot handles no-index headers very elegantly. It advises to leave as many routes as possible open and uses headers for high fidelity rules regarding indexes.
Discourse adds special `x-robot-tags` noindex headers to users, badges, groups, search and tag routes.
Following up on b52143feff8c32f2 we now have it so Googlebot gets special handling.
Rest of the crawlers get a far more aggressive disallow list to protect against excessive crawling.