mail-addons/mail_all/static/src/xml/menu.xml

<?xml version="1.0" encoding="UTF-8" ?>
<template>
    <!--Inherit Sidebar and add All messages menu item after Starred -->
    <t t-extend="mail.chat.Sidebar">
        <t t-jquery="div[data-channel-id=channel_starred]" t-operation="after">
            <div
                t-attf-class="o_mail_chat_title_main o_mail_chat_channel_item #{(active_channel_id == 'channel_all') ? 'o_active': ''}"
                data-channel-id="channel_all"
            >
                <span class="o_channel_name mail_all">
                    <i class="fa fa-database" />
                    All messages
                </span>
            </div>
        </t>
    </t>
    <!--Add message about empty all messages page-->
    <t t-extend="mail.EmptyChannel">
        <t t-jquery="t:last-child" t-operation="after">
            <t t-if="options.channel_id==='channel_all'">
                <div class="o_thread_title">No messages</div>
            </t>
        </t>
    </t>
</template>