The second result google result for 'maniphest custom fields' is: https://secure.phabricator.com/book/phabricator/article/maniphest_custom/
Which just has This atom no longer exists. That's seems not intentionl for docs? Either a redirect or 404 to delist would probably get people to the right place sooner. Maybe related to D13114?
In this case, we should maybe have a way to put a redirect in Diviner, since this atom has legitimately moved here:
https://secure.phabricator.com/book/phabricator/article/custom_fields/
In the more general case of a removed atom, I think changing the HTTP response code to 404 is reasonable, although these stubs exist partially because there may be useful information on the page in the future (comments, access to old versions, whatever), and it might not make sense to 404 them anymore once that stuff is added.
I'm just going to put a redirect map in the book for now. This isn't a great long-term solution but I want to move forward on D13941 in some form.
Ugh, it's too hacky. I'm just going to put a manual redirect in place for now instead.