This is a key requirement for multi-channel Program use cases:

Contacts that are opted out of email can be entered into a Program, however it isn't possible to query 'Opted Out = T' in a decision point in order to route individuals to different paths where they will receive messages through the channel they are opted into (1 path for email, 1 path for SMS, 1 path for mobile for example). Instead they are exited from the Program as soon as they are routed to an Email step.