register_post_type() has a argument that you can set to specify which post types you want searched, thus excluded the undesired ones. This is the simplest method, just one line:
'exclude_from_search' = true,
Alternative to that, you can hook into the search query itself via pre_get_posts and then modify the search, specifying which post types you’re after:
add_filter('pre_get_posts',function ($query) {
if ($query->is_search && !is_admin() )
$query->set('post_type',array('post','page'));
return $query;
});
If you were dead set on doing the term hiding – although I don’t recommend it, as it’s more code with a higher chance of user error – you’d do somthing like:
add_action( 'pre_get_posts', function ( $query ) {
global $wp_the_query;
if($query === $wp_the_query && $query->is_search() && !is_admin()) {
$tax_query = array(
array(
'taxonomy' => 'your_custom_tax_name',
'field' => 'slug',
'terms' => 'hidden',
'operator' => 'NOT IN',
)
);
$query->set( 'tax_query', $tax_query );
}
});