I've been asked to implement 2FA with email codes, like you get from Steam (and many banks), after you haven't logged in for awhile. I initially thought this would have been a flag I could turn on in the Devise config, but I can't find ANY place on the internet that talks about something like this. The desired process would be to generate and email a one-time pad to enter into a confirmation screen. Every reference I've found to 2FA with Devise refers to using things like SMS or an authenticator app.
Working within the framework of Devise, it seems like this might possibly boil down to unconfirming the user every so often, maybe like every other day. That way, the next time they log in, they get another email with a new link to "re-"confirm the login. The best I can find is Warden::Manager.after_authentication to set user.confirmed_at = nil, but this doesn't seem to be doing what I want.
Thanks to a friendly person on Github, I was directed to the Devise plugin, https://github.com/Houdini/two_factor_authentication, which does exactly what I wanted. I knew someone had to have already written it!