social/mail_optional_follower_notification/wizard/mail_compose_message_view.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data noupdate="0">
        <record model="ir.ui.view" id="email_compose_message_wizard_inherit_form">
            <field name="name">mail.compose.message.form (mail_optional_autofollow)</field>
            <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" />
                </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>
    </data>
</odoo>