Prevent Redactor from adding site identifier to entry links
I solved this by creating a Redactor plugin that listens for the link.inserted event and strips out the site identifer:

craft/config/redactor/plugins/myplugin.js:

(function($R)
{
    $R.add('plugin', 'myplugin', {
        onlink: {
            inserted: function(link) { 
                link.nodes.forEach(node =&gt; node.href = node.href.replace(/@\d+/, ''))
             },
        },
    });
})(Redactor);


craft/config/redactor/redactorFieldConfig.json:

{
    "plugins": ["myplugin"]
}