The RX7 is in the game files along with 4 other cars. The car along with other missing cars are confirmed as an update. Ghost has stated that they need more work to make the cars properly functioning. You can see several videos on YouTube about the missing cars being in the game files and being used as driveable cars. However, don't expect to get any information about getting them yourself before the update as they are only demonstrating and are most probably bound by contracts. From the videos, you can notice that they are unstable or weird to drive and need more work.
Just have patience and trust Ghost. They really care about the players and are trying to fix the franchise.