If you can open a ticket (via support at namecheap dot com) with details and the domain name, I'll make sure this gets appropriately escalated. Just let me know the ticket number when you do.
You have brought up what is precisely my concern, and what I couldn't convey to support.
If I open a ticket with the domain name, what's to stop the access being removed, and declaring "problem solved", without actually addressing the underlying issue?
Either delegated access remaining after a sale is a mistake (in which case there is now "unauthorized access"), or it's "by design" (in which case users should be made aware so they can remove this access after buying a domain). Regardless, I think you shouldn't need to know the specific domain in order to say which situation it is.
Support is aware of your issue and is awaiting details, and this is now in a specific escalation process to upper management. I will need some information in order to further assist you, especially to understand why this is impacting you and/or if it could be impacting others. This does not sound like something that is "by design." If you don't own or manage the domain, it shouldn't be listed in your account. Period. But we can't address any such underlying issue without a starting point.
You don't need to tell me the domain here. Please simply open a ticket and let me know the ticket number and I will have management take a closer look.