mail-addons/mail_private/full_composer_wizard.xml

<?xml version="1.0" encoding="utf-8" ?>
<!--Copyright 2017 Artyom Losev <https://github.com/ArtyomLosev>
    Copyright 2019 Artem Rafailov <https://it-projects.info/team/Ommo73/>
    License MIT (https://opensource.org/licenses/MIT).-->
<odoo>
    <record model="ir.ui.view" id="email_compose_message_wizard_form_private">
        <field name="name">mail.compose.message.form.private</field>
        <field name="model">mail.compose.message</field>
        <field name="groups_id" eval="[(4,ref('base.group_user'))]" />
        <field name="inherit_id" ref="mail.email_compose_message_wizard_form" />
        <field name="arch" type="xml">
            <data>
                <xpath expr="//field[@name='active_domain']" position="after">
                    <field name="is_private" invisible="1" />
                </xpath>
                <xpath
                    expr="//div[@groups='base.group_user']/span[2]"
                    position="attributes"
                >
                    <attribute name="attrs">{'invisible': [('is_private', '=', True)]}
                        </attribute>
                </xpath>
            </data>
        </field>
    </record>
</odoo>