Site ownership is a self-service feature, and Pantheon does not transfer ownership from one owner to another. Best practice is to own your site in your name with a permanently reachable email.
This article will help you gain access to an account if a site owner has left the agency or project, and you do not have the login information.
If the site is associated with an Organization that has a Pantheon Account Manager, you will need to contact that person in order to request a Support ticket for further assitance. For security and privacy reasons, if the site does not have an Account Manager, Pantheon cannot intercede and forcibly transfer ownership of a site from one user to another.