Is the remaining numbness in the chin/lower lip (a year after jaw surgery and sliding genioplasty) permanent, or could it still improve?

Most Likely Will Not: Although very rare, I have seen patients who have had sensation recovery beyond a year. There are several injuries to a nerve that can occur. Neuropraxia, axonotmesis, and neurotmesis. Best to discuss this with your oral and maxillofacial surgeon.

Yes: It is unlikely to return to normal this late after surgery but it could still improve, especially if you have noticed any return over the last several months. Tingling is a sign of healing.