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 LGPL-3.0 (https://www.gnu.org/licenses/lgpl.html).-->
<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>