social/mail_optional_follower_notification/wizard/mail_compose_message_view.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <record id="email_compose_message_wizard_form" model="ir.ui.view">
        <field name="model">mail.compose.message</field>
        <field name="inherit_id" ref="mail.email_compose_message_wizard_form"/>
        <field name="arch" type="xml">
            <xpath expr="//div[field[@name='partner_ids']]/span[2]" position="before">
                <field name="notify_followers" attrs="{'invisible': [('composition_mode', '=', 'mass_mail')]}"/>
            </xpath>
            <xpath expr="//div[field[@name='partner_ids']]/span[2]" position="inside">
                <span attrs="{'invisible': [('notify_followers', '=', True)]}" style="color: red;"> - Warning : Followers will not be notified but they can access the notification directly from the document (if they are allowed to)</span>
            </xpath>
        </field>
    </record>

</odoo>