Sadly I don’t find any mention of this in the documentation. So is this only there for the case that a released app is open source? Or is this for something else?
I also couldn’t find any app providing such a link on the marketplace, which suggests that not many devs or dev teams offer such an agreement.
The idea behind that URL is that, similar to Atlassian, Marketplace Partners can decide that they allow users access to the source code.
This can be either through an open source release (for instance on GitHub) or by providing access to a ZIP file (upon request or through a customer portal). If this option is available to customers, you can provide the URL to the policy that governs such access to the (closed) source.
Usually the page to which this link refers has information to how customers are supposed to use the source code as well as includes information on how the customer can get access to the source code, for instance through a support request.