Questions have been asked about the length of time that Kililo Kijakazi has served as Acting Commissioner of Social Security. The Vacancies Reform Act places limits on the length of time that a person can serve in an acting capacity as head of a federal agency. The contrary argument has been that the Vacancies Reform Act doesn't apply to an Acting Commissioner of Social Security since the Social Security Act itself has a specific provision concerning the designation of an Acting Commissioner which supersedes the Vacancies Reform Act. For whatever reason, the job of deciding how the Vacancies Reform Act applies to various positions has been lodged with the Government Accountability Office (GAO). The GAO has now issued an official ruling on the question. Kijakazi can continue to serve as Acting Commissioner because the Vacancies Reform Act doesn't apply to Social Security.
The GAO ruling, of course, gives no answer to the question of why the President hasn't nominated a Commissioner. My guess is that because of the firing of Andrew Saul that Republicans would mount a pitched battle against the confirmation of anyone as Commissioner and the President, or more likely Senate Democratic leaders, don't want to waste the time on it.
I don't like the idea that only Republicans can be confirmed as Commissioner of Social Security.