What future plan about Israel and the Gentiles was still a secret at the time of Acts 1?
At the time of Acts 1, the secret plan was that Israel would reject the offered kingdom, and God would turn to the Gentiles under a whole different program. That transition had not yet been revealed; God kept it hidden until His appointed time to reveal it.
Les explains that Jesus and the eleven were still on the Mount of Olives, talking about things pertaining to the kingdom. The kingdom was valid and real, not a figment of their imagination; the only thing not for them to know was the timing. Jesus knew that Israel would reject all this and that He would be turning to the Gentiles under a whole different program. But that plan was secret, and He had not revealed it yet. Les notes that God has the prerogative to keep things secret until He is ready to reveal them.