The only place I've seen this done is Google. For them it makes sense. Since a long time ago, they've had a feature where you use your custom authentication se

The only place I've seen this done is Google. For them it makes sense. Since a long time ago, they've had a feature where you use your custom authentication service to sign in (think: on site Kerberos instance). For example at school the form would show both email and password fields, but I would enter only my school email into the username and then it would redirect me to to my school's centralized login. So now instead of mistakenly showing the password field sometimes, they only show it when necessary.