I'd like to change the default page that is served when a user navigates to a donation form that is no longer published. How can I do this?
There is a site setting that will set the global redirect URL for unpublished donation forms. Any URL specified here will result in the page referenced by the URL being served instead of the default campaign list page. If no value is specified, then the campaign list page will be served. If you set an Unpublished URL value on the donation form, that URL will override this site setting.