It is safe to assume that if a user explicitly set their user digest setting, then they can consider having opted into the email.
For those users that have not set any setting explicitly, you can change the default in the ACP, and that value will propagate to those users as well. So you'll probably want to disable the email digest for default users.
I will try that for now
Problem with the new law is that users need to re-confirm his preferences and if they dont confirm it should be disabled
I'd like to be able to decide what categories users are automatically subscribed to. I know there's a plugin to prevent new users from automatically subscribing to all categories, but I'd prefer a lot more granularity than that.