Alaska, Omicron Major/Minor, and the Tohoku systems are closed off after you're finished with the story. You will get an "access denied" message trying to use the jumpgates or jumpholes that lead to those systems. And if you wander too far away from your waypoints to go exploring while you're in those systems on a storyline mission, you risk failing the mission.

I suspect there isn't anything else to find, anyway... but you never know... maybe someone will create an "access granted" mod for those systems.