Add "Reviewed By" and Last Reviewed Date to Yoast's WebPage Schema

// Post types that get the reviewer box.
$wpide_review_types = array( 'post', 'page' );

add_action( 'add_meta_boxes', function () use ( $wpide_review_types ) {
    add_meta_box( 'wpide-reviewed-by', 'Expert review', function ( $post ) {
        wp_nonce_field( 'wpide_review_save', 'wpide_review_nonce' );
        $reviewer = (int) get_post_meta( $post->ID, '_wpide_reviewer', true );
        $date     = get_post_meta( $post->ID, '_wpide_reviewed_on', true );

        echo '<p><label for="wpide-reviewer">Reviewed by</label><br>';
        wp_dropdown_users( array( 'name' => 'wpide_reviewer', 'id' => 'wpide-reviewer', 'selected' => $reviewer, 'show_option_none' => '— None —', 'option_none_value' => 0, 'capability' => array( 'edit_posts' ) ) );
        echo '</p><p><label for="wpide-reviewed-on">Last reviewed</label><br>';
        printf( '<input type="date" id="wpide-reviewed-on" name="wpide_reviewed_on" value="%s"></p>', esc_attr( $date ) );
    }, $wpide_review_types, 'side' );
} );

add_action( 'save_post', function ( $post_id ) use ( $wpide_review_types ) {
    if ( ! isset( $_POST['wpide_review_nonce'] ) || ! wp_verify_nonce( sanitize_key( $_POST['wpide_review_nonce'] ), 'wpide_review_save' ) ) {
        return;
    }

    if ( wp_is_post_autosave( $post_id ) || ! in_array( get_post_type( $post_id ), $wpide_review_types, true ) || ! current_user_can( 'edit_post', $post_id ) ) {
        return;
    }

    $reviewer = isset( $_POST['wpide_reviewer'] ) ? absint( $_POST['wpide_reviewer'] ) : 0;
    $date     = isset( $_POST['wpide_reviewed_on'] ) ? sanitize_text_field( wp_unslash( $_POST['wpide_reviewed_on'] ) ) : '';

    update_post_meta( $post_id, '_wpide_reviewer', $reviewer );
    update_post_meta( $post_id, '_wpide_reviewed_on', preg_match( '/^\d{4}-\d{2}-\d{2}$/', $date ) ? $date : '' );
} );

add_filter( 'wpseo_schema_webpage', function ( $data ) {
    if ( ! is_array( $data ) || ! is_singular() ) {
        return $data;
    }

    $post_id  = get_queried_object_id();
    $reviewer = (int) get_post_meta( $post_id, '_wpide_reviewer', true );
    $date     = get_post_meta( $post_id, '_wpide_reviewed_on', true );

    if ( $reviewer && get_userdata( $reviewer ) ) {
        $data['reviewedBy'] = array(
            '@type' => 'Person',
            'name'  => get_the_author_meta( 'display_name', $reviewer ),
            'url'   => get_author_posts_url( $reviewer ),
        );
    }

    if ( $date ) {
        $data['lastReviewed'] = $date;
    }

    return $data;
} );