You will never find a controlled frequency response graph of a headphone before burn in and after burn in showing any difference, because there isn't.

What does exist:

Brain burn in - where you get used to how a headphone sounds and start to notice the intricate details it can present, or the low end that you thought was lacking but is actually just a little recessed but better quality than your previous headphones.

Pad burn in/wearing out - if you use a headphone for a long time, the pads will wear down which will change the sound of the headphones. It is not the driver itself burning in.

Changing how you wear the headphones - if you increase or reduce the clamp, start wearing glasses, have a lot of hair cut off or grow a lot of hair, these can all change how your headphones sound.