sp-migration-info.js

function addMigrationInfo(sender, args) {
    SP.SOD.executeFunc('sp.js', 'SP.ClientContext', function() {
        var statusId = SP.UI.Status.addStatus("This site has moved. Please update you bookmarks and visit the new location: <a href='" + url + "'>" + url + "</a>");
        SP.UI.Status.setStatusPriColor(statusId, 'yellow');
    });
}

// Initialize the statusbar as late as possible, see: https://blog.josequinto.com/2015/06/16/custom-javascript-function-loaded-after-the-ui-has-loaded-in-sharepoint-2013/
Sys.WebForms.PageRequestManager.getInstance().add_pageLoaded(addMigrationInfo);