A diretiva noindex é entregue tanto no cabeçalho da resposta quanto no arquivo robots.txt:
➜ ~ curl https://meta.discourse.org/tag/planned -I -X GET
HTTP/2 200
date: Tue, 09 Feb 2021 14:38:52 GMT
...
x-robots-tag: noindex
...
➜ ~ curl https://meta.discourse.org/u/Falco -I -X GET
HTTP/2 200
date: Tue, 09 Feb 2021 14:39:38 GMT
...
x-robots-tag: noindex
...
https://meta.discourse.org/robots.txt
User-agent: *
...
Disallow: /u
Disallow: /my
Disallow: /tag
...