mass_mailing/data/digest_data.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data>
        <template id="digest_mail_main" inherit_id="digest.digest_mail_main">
            <xpath expr="//div[hasclass('by_odoo')]/t[last()]" position="after">
                <t t-if="mailing_report_token">
                    –
                    <a t-attf-href="/mailing/report/unsubscribe?token=#{mailing_report_token}&amp;user_id=#{user_id}"
                       target="_blank" style="text-decoration: none;">
                        <span style="color: #8f8f8f;">Turn off Mailing Reports</span>
                    </a>
                </t>
            </xpath>
        </template>
    </data>
</odoo>