It's in development with the Pro box getting first release, the Discovery+ app is free of charge, it was given to all BT Sport subscribers at no extra cost.
The Discovery channel was also given at no extra cost to BT TV customers with Entertainment, Big Entertainment and VIP packages.
The statement from BT on its web page says
discovery+ is not yet available to watch on BT TV boxes. We expect the discovery+ app to launch on our TV Box Pro and TV Box Mini in the future,
As for the entitlement it says
All BT Sport subscribers who subscribe via BT, EE or Plusnet will be entitled to activate a discovery+ Entertainment & Sport subscription. Your access will continue for as long as you are subscribed to BT Sport or until the discovery+ Entertainment & Sport plan comes to an end.