For the shortcode try the post_types attribute:
https://keesiemeijer.wordpress.com/related-posts-by-taxonomy/#post-types
[related_posts_by_tax post_types="page"]
And for the widget try it with this in your theme’s functions.php file. It adds a ‘page’ checkbox to the widget.
add_filter( 'related_posts_by_taxonomy_widget_form_fields', 'add_post_type_page_related_widget', 10, 3 );
function add_post_type_page_related_widget( $pieces, $i, $_this ) {
$post_types = $_this->defaults->post_types;
$post_types['page'] = __('Page');
$after = "\n\t";
$before = "\t";
// post types
$field = '<div class="rpbt_post_types"><h4>' . __( 'Post Types', 'related-posts-by-taxonomy' ) .' </h4><p>';
foreach ( $post_types as $name => $label ) {
$field .= '<input type="checkbox" class="checkbox" id="' . $_this->get_field_id( 'post_types' ) . "_$name" . '" ';
$field .= 'name="' . $_this->get_field_name( 'post_types' ) . "[$name]" .'"';
if ( isset( $i['post_types'][ $name ] ) && ( 'on' === $i['post_types'][ $name ] ) ) {
$field .= ' checked="checked"';
}
$field .= ' /> <label for="' .$_this->get_field_id( 'post_types' ) . "_$name" . '">' . $label . '</label><br />';
}
$pieces['post_types'] = $before . $field . '</p></div>' . $after;
return $pieces;
}
btw:
consider creating a child theme instead of editing your theme directly – if you upgrade the theme all your modifications will be lost. Or create a plugin with the code above.