Yoast SEO: register extra replacement to get content before read more tag

wpseo-register-replacement-for-read-more.php

<?php
add_action(
    'wpseo_register_extra_replacements',
    /**
     * Yoast SEO: register extra replacement to get content before read more tag
     */
    function () {
	    wpseo_register_var_replacement(
            '%%read_more%%',
            function () {
                return wp_strip_all_tags( get_extended( get_post()->post_content )['main'] );
            },
            'advanced',
            'Get content before read more tag'
        );
    }
);