He hasn't regressed. The league has figured him out.
He made lots of plays last year OUTSIDE THE POCKET. The Vikings and Giants showed everybody what to do. Crowd the line of scrimmage, take away the short passes, and don't let him get outside the pocket. He will not hurt you if you keep him inside the pocket. He has not confidence to throw from there downfield.
Now, it's up to him to change that.
Bad defenses will still get torched by Dak and this offense, but good defenses can shut it down. He doesn't know how to pick apart coverages and blitz schemes like Romo learned to do, so he's just fish in a barrel for defensive coordinators right now.