While usernames are easy to figure out, I think email divulgation should be taken more seriously. Even a registration form should not inform you that the email

While usernames are easy to figure out, I think email divulgation should be taken more seriously. Even a registration form should not inform you that the email is already used. This information could be given ... by sending a mail. While disclosure of the existence of an account with your email have minimal impact on Stripe or GitHub, what about a dating site ? What about a politic website ? How about some sensible professional tool ?