Show all terms of a custom taxonomy?

You need to pass an additional argument to get_terms(). The default is to hide “empty” terms– terms which are assigned to no posts.

$terms = get_terms([
    'taxonomy' => $taxonomy,
    'hide_empty' => false,
]);

EDIT:

Incase you want to display the name or the slug of the enlisted custom taxonomies being held by the $terms variable you can use this piece of code bellow:

foreach ($terms as $term){
  echo $term->slug." : ";
  echo $term->name;
  echo "<br><br>";
}

Where $term->slug outputs the slug of the taxonomy item that’s enlisted and $term->name outputs the name of the according taxonomy item.