Allow number slug in page url

functions.php

<?php

/**
 * Allow number slug in page url
 *
 * Since WordPress do not allow number links in url we must generate it using
 * `wp_unique_post_slug` filter.
 *
 * @link https://github.com/WordPress/WordPress/blob/4.9.2/wp-includes/post.php#L3798
 *
 * `preg_match( "@^($wp_rewrite->pagination_base)?\d+$@", $slug )`
 * `preg_match( "@^(page)?\d+$@", $slug )`
 *
 * @param string $slug          The post slug.
 * @param int    $post_ID       Post ID.
 * @param string $post_status   The post status.
 * @param string $post_type     Post type.
 * @param int    $post_parent   Post parent ID
 * @param string $original_slug The original post slug.
 *
 * @return string
 */
function motivast_allow_number_post_slug( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ) {

    global $wpdb, $wp_rewrite;

    $number_slug = $original_slug;

    // If this condition is not true this is not our number slug case
    if( ! preg_match( "@^($wp_rewrite->pagination_base)?\d+$@", $original_slug ) ) {
        return $slug;
    }

    $check_sql = "SELECT post_name FROM $wpdb->posts WHERE post_name = %s AND post_type = %s AND ID != %d LIMIT 1";
    $post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $original_slug, $post_type, $post_ID ) );

    // If post with this name already exits generate new one with suffix
    if( $post_name_check ) {

        $suffix = 2;

        do {
            $alt_post_name = _truncate_post_slug( $number_slug, 200 - ( strlen( $suffix ) + 1 ) ) . "-$suffix";
            $post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $alt_post_name, $post_type, $post_ID ) );
            $suffix++;
        } while ( $post_name_check );

        $number_slug = $alt_post_name;
    }

    return $number_slug;
}

add_filter( 'wp_unique_post_slug', 'motivast_allow_number_post_slug',  10, 6 );

/**
 * Return empty date rewrites rules to not conflict with our new number format
 *
 * Remember to regenerate permalinks structure.
 *
 * @param array $date_rewrite The rewrite rules for date archives.
 *
 * @return array
 */
function motivast_date_rewrite_rules( $date_rewrite ) {
    return array();
}

add_filter( 'date_rewrite_rules', 'motivast_date_rewrite_rules', 10, 1 );