Activate "Activités de publication" for all new page items by default. (Requires https://github.com/imath/activites-de-publication/)

bp-custom.php

<?php
/**
 * For more information about how to use the bp-custom.php file
 * @see https://codex.buddypress.org/themes/bp-custom-php/
 */

// Exit if accessed directly
defined( 'ABSPATH' ) || exit;

/**
 * Activate "Activités de publication" for all new page items by default.
 *
 * @param  boolean       $show_in_rest True by default.
 * @param  string        $post_type    The post type name.
 * @return boolean|array               Unchanged argument or an array containing
 *                                     the metadata schema default value.
 */
function imath_register_meta_rest_attributes( $show_in_rest = true, $post_type = '' ) {
    if ( 'page' === $post_type ) {
        $show_in_rest = array(
            'schema' => array(
                'default' => true,
            ),
        );
    }

    return $show_in_rest;
}
add_filter( 'post_activities_register_meta_rest_attributes', 'imath_register_meta_rest_attributes', 10, 2 );