They can either maintain it themselves, or let another party do it for them, indeed. The contracts specify under which circumstances the code can be released, but usually it's going out of business or cancelling a product.

It's something we had to deal with in a previous job where we (as a small company) sold products to Global 500 companies. Either we agreed to this, or there would be no deal. Every time we released a new version to those clients, we would place the updated source code (along with documentation and tooling etc) in escrow.