Splitting this out from bug 46491.
It's been pointed out that the current behavior of user pages of users who exist, but who do not have a user page (e.g., https://test.wikipedia.org/wiki/User:Red_Baron) is a little strange/questionable. There's a generic "this site does not have a page with that name" error that's output, but it's perhaps more logical to output basic info about the user (while simultaneously marking the page as "noindex").
I think this has the potential to upset some users, but it would probably be better than the current behavior. For example, there's a garish red box that appears for users with a deleted user page (cf. https://en.wikipedia.org/wiki/User:JzG).