With most large cabin business jets, having a cockpit door is strictly up to the owner. On our Falcon 900EX we had a door between the galley and the cabin and a door between the cockpit and the galley.
For the aircraft to be certified with the cockpit doors, they had to placarded "To Remain Open for Takeoff and Landing."
However, even if left closed, they can easily be kicked or pushed open in case of an accident.
I also have flown Sabreliners and Falcon 50s with cockpit doors.