@stufromoz @atomicpoet @MsPraxis That and this.
Now, these don't apply very well to web search engines because they just see the *web* sites humans see, so for example a thread page with your reply may still be indexed because only the whole page can be opted-out (mastodon sets this based on the selected post of the page i think).
But an ActivityPub search engine would have the ability to fully honor them.