As a desktop email client developer, I'd like to support OAuth2 for Yahoo IMAP, but apparently there is no way to get permission for the scopes needed for this. Given that a client id cannot be kept as a secret on desktop machines (for other email clients) then it doesn't make much sense to not allow this. It's just security by obscurity and you risk that email clients borrow client ids from other email clients (I'll naturally not do that).

This is where a note about how to apply would be a nice addition: https://developer.yahoo.com/oauth2/guide/yahoo_scopes/