dropdown event filter in hierarchical manner?

I would also need this

// Change to event.module for hierarchial dropdown in display

From:
foreach ($tree as $term) {
$items[$term->tid] = $vocab->name .' - '. $term->name;
}

To:
foreach ($tree as $term) {
$dash = '';
while ($term->depth--) {
$dash .= '-';
}
$items[$term->tid] = $dash . $term->name;
}

You'll have to arrange the categories the way you want it.
This just displays them the way they are categorized, like in Create Event.

Main
-Sub Category
--Sub Sub

Event for Drupal 8 is unrelated to older versions. If an issue similar to this one exists, please open a new issue with the 8.x branch.