example.php

<?php
// Example meta box field
$cmb->add_field( array(
	'name'           => 'Test Select',
	'id'             => 'test_select',
	'type'           => 'pw_multiselect',
	'options_cb'     => 'iweb_get_cmb2_term_options',
	'get_terms_args' => array(
		'taxonomy'   => 'post_tag',
	),
) );

/**
 * Get a list of terms.
 *
 * Generic function to return an array of taxonomy terms formatted for CMB2.
 * Simply pass in your get_terms arguments and get back a beautifully formatted
 * CMB2 options array.
 *
 * Usage:
 *
 *    $cmb->add_field( array(
 *        'name'           => 'Test Select',
 *        'id'             => $prefix . 'test_select',
 *        'type'           => 'select',
 *        'options_cb'     => 'iweb_get_cmb2_term_options',
 *        'get_terms_args' => array(
 *            'taxonomy'   => 'post_tag',
 *        ),
 *    ) );
 *
 * @param  CMB2_Field $field The field object.
 *
 * @return array An array of options that matches the CMB2 options array.
 */
function iweb_get_cmb2_term_options( $field ) {
	$defaults = array(
		'taxonomy'   => 'category',
		'hide_empty' => false,
	);

	$args = $field->args( 'get_terms_args' );
	$args = is_array( $args ) ? $args : array();
	$args = wp_parse_args( $args, $defaults );

	$terms = get_terms( $args );

	$term_options = array();

	if ( ! empty( $terms ) ) {
		$terms_with_hierarchy = iweb_build_term_tree( $terms );
		$term_options         = wp_list_pluck( $terms_with_hierarchy, 'name', 'term_id' );
	}

	return $term_options;
}

/**
 * Sort terms into a tree structure to show term hierarchy.
 *
 * In order to show hierarchical terms, we recursively build an options array,
 * prepending the term title with a depth indicator to give the term some
 * context.
 *
 * @param array $terms     Array containing WP_Term objects.
 * @param int   $parent_id The current parent.
 * @param int   $level     The current level.
 *
 * @return array
 */
function iweb_build_term_tree( $terms, $parent_id = 0, $level = 0 ) {
	$branch = array();

	foreach ( $terms as $term ) {
		if ( $parent_id == $term->parent ) {
			$term->name = str_repeat( '&mdash; ', $level ) . $term->name;

			$branch[] = $term;

			$children = iweb_build_term_tree( $terms, $term->term_id, $level + 1 );
			if ( ! empty( $children ) ) {
				foreach ( $children as $child ) {
					$branch[] = $child;
				}
			}
		}
	}

	return $branch;
}

I think it would be better to use a callback as mentioned here https://cmb2.io/docs/field-parameters#-options_cb

This would prevent the function to fire at every page load.

@eb-ai4 You're right. I've updated the gist with a later version of this helper function. This version also better represents hierarchical taxonomies.