I hate to be the bearer of bad news, but this feature request has been declined.
Yeah this is a ridiculous decision. The early OneNote tabs didn't have the ability to add Sections. Now I can add new OneNote notebooks that have multiple tabs, or even attach existing OneNote notebooks, but this old immovable tab I can't get rid of! This is terrible customer/product support and sure does reduce the incentive to use Teams if this is the kind of support we'd expect.