Almost all the Rails apps I worked on in the past 18 years don't care about SEO because they don't have much that googlebot can browse without an account. Every

Almost all the Rails apps I worked on in the past 18 years don't care about SEO because they don't have much that googlebot can browse without an account. Everything important is either behind a login form or served through an API. The API is probably used by a JS frontend, which googlebot browses, and it's up to the frontend to deal with 404 and other errors in a gracious way. Finally, if the API is machine to machine, there are no worries about SEO.