@terryenglish there should be a site-wide setting for Mastodon in the UI, and a question at setup-time for Pleroma.

I'm not aware of any per-account settings. These would be counter-productive, since the users wishing to be excluded from indexing would then show up in the robots.txt.
A fallacy which the FBI also learned about, lol. (They had paths to non-public sites in robots.txt, thus making them public)

robots.txt works against google, bing, yandex crawlers. People who have doubts about that need to get a clue or/and conduct their own tests.