Our add-in/app includes a document library instance used for storing customisable image assets. A few months ago the "Exit classic experience" link started appearing at the top of the list view pages, however the link does nothing. I thought it was a glitch that would go away, however it persists even in newly-installed copies of our add-in, and a recent bug-fix we submitted to Microsoft just got rejected because the link made the add-in "look incomplete". We had to remove all UI references and page links to the library, as a measure to prevent the testers from seeing the offending link.
Why does this link appear here, and what can we do to remove it? We are using classic Declarative XML (elements.xml) to define the library instance (ListInstance), provisioning it within the App Web. We did not write our own template, and the ListInstance schema doesn't seem to provide access to any properties that would show/hide the Modern Experience. We do not define the list view pages ourselves, and so we do not seem to have the abilty to serve CSS to simply hide the link. And App Web means we do not have access to a List Settings or Site Settings page.