add_filter( 'get_terms_fields', function( Array $selects, Array $args, Array $taxonomies )
{
if ( ! in_array( 'tax_name', $taxonomies ) )
return $selects;
remove_filter( current_filter(), __FUNCTION__ );
$selects[] = 't.slug';
return $selects;
}, 10, 3 );
$terms = get_terms( 'tax_name', [ 'fields' => 'names', ] );