Is the voice cast section of the SU Future page the entire official cast that will be appearing on the show? (As it features characters like Spinel and the Diamonds who haven’t even appeared yet). Or is it still a work in progress that will be updated as more characters appear in the series?
It's a work in progress as the series is not over, who knows if there will be new characters coming ¯\_(ツ)_/¯. It does feature characters that have yet to appear like you said, characters like Spinel and the Diamonds, which only appeared in the opening.
Think this thread is in the wrong board as it's a question anyone can answer and the Crewniverse rarely look at wikia or have accounts.