What can be done for a 20 year old with untreated plagiocephaly?

Not much: You are trying to alter the shape of the skull. You should go to a center for craniotomy-facial disorders and they will guide you into what, if anything, is possible. I hope you have great medical insurance or are wealthy.

Accept it/ change it: Reshaping a skull requires rather dramatic surgery. They pull back the scalp like a cover, cut up the skull bones in sections & reposition them. The preparation requires a variety of 3D MRI scans, hours in the OR and days in the ICU afterward, with a prolonged recovery.If you are interested contact a regional craniofacial center and plan to spend big bucks. Insurers are unlikely to cover it.

Not much: The doctors who deal with those issues are at cranio-facial centers. They have one at NYU and also at Montefiore in NYC. You are kind of old for this but at least you can talk to them about it.