Replacement with categories/tags as checkboxes and hierarchical indents

taxonomy.php

<?php
// Don't load directly
defined( 'WPINC' ) or die;

$selected_terms = array();
$taxonomy_obj   = get_taxonomy( $taxonomy );
$ajax_args = array(
	'taxonomy' => $taxonomy,
);

$taxonomy_label = $taxonomy_obj->label;

// If we already have Event on the actual Taxonomy Label
if ( false === strpos( $taxonomy_obj->label, tribe_get_event_label_singular() ) ) {
	$taxonomy_label = sprintf( '%s %s', tribe_get_event_label_singular(), $taxonomy_obj->label );
}

// Check we have terms
$has_terms = count( get_terms( $taxonomy, array( 'hide_empty' => false, 'number' => 1, 'fields' => 'ids' ) ) ) < 1;

// Setup selected tags
$value = ! empty( $_POST['tax_input'][ $taxonomy ] ) ? explode( ',', esc_attr( trim( $_POST['tax_input'][ $taxonomy ] ) ) ) : array();

// if no tags from $_POST then look for saved tags
if ( empty( $value ) ) {
	$terms = wp_get_post_terms( get_the_ID(), $taxonomy );
	$value = wp_list_pluck( $terms, 'term_id' );
}

foreach ( $value as $term_id ) {
	$term = get_term( $term_id, $taxonomy );
	$selected_terms[ $term_id ] = array(
		'id'   => $term->term_id,
		'text' => $term->name,
	);
}

if ( is_array( $value ) ) {
	$value = implode( ',', $value );
}

if ( $has_terms ) {
	return;
}

$hierarchical_terms_list = function( $taxonomy, $term_parent = 0, $depth = 0 ) use ( &$hierarchical_terms_list ) {

	$terms = get_terms( array(
		'taxonomy' => $taxonomy,
		'get'      => 'all',
		'parent'   => $term_parent
	) );

	if ( empty( $terms ) ) {
	    return array();
    }

	foreach ( $terms as $single_term ) {
		$single_term->depth = $depth;
		$term_list[] = $single_term;

		$children = $hierarchical_terms_list( $taxonomy, $single_term->term_id, $depth + 1 );

		if ( is_array( $children ) ) {
			$term_list = array_merge( $term_list, $children );
		}
	}

	return $term_list;
};
?>
<div class="tribe-section tribe-section-taxonomy">
	<div class="tribe-section-header">
		<h3><?php echo esc_html( $taxonomy_label ); ?></h3>
	</div>

	<?php
	/**
	 * Allow developers to hook and add content to the beginning of this section
	 * @param string $taxonomy_slug
	 */
	do_action( 'tribe_events_community_section_before_taxonomy', $taxonomy );
	?>

	<?php foreach ( $hierarchical_terms_list( $taxonomy ) as $term ): ?>
		<label>
			<input
				type="checkbox"
				name="tax_input[<?php echo esc_attr( $taxonomy ); ?>][]"
				value="<?php echo esc_attr( $term->term_id ); ?>"
				<?php echo isset( $selected_terms[ $term->term_id ] ) ? 'checked="checked"' : ''; ?>
				style="margin-left: <?php echo esc_attr( $term->depth * 1.75 ); ?>rem"
			/>
			<?php echo esc_html( $term->name ); ?>
		</label> <br/>
	<?php endforeach; ?>

	<?php
	/**
	 * Allow developers to hook and add content to the end of this section
	 * @param string $taxonomy_slug
	 */
	do_action( 'tribe_events_community_section_after_taxonomy', $taxonomy );
	?>
</div>

Hacky! Moving the logic used to get the term depth out from the template and something cleaner than setting the left margin via the style attribute would be nicer.