How can I bulk delete Menu Itemes in Drupal 8
Bulk delete

there is module https://github.com/robdecker/menu_delete which you can use that provides checkboxes for easier delete of menu items. 

its at the time of this writing missing a SELECT ALL feature which would make it even more helpful

its proposed to be included in core but there is no sign that it will https://www.drupal.org/node/2772961

so you can install it and take advantage of it using following commands

cd modules
git clone https://github.com/robdecker/menu_delete.git
drush en menu_delete -y


and then go into your menu and you should have available checkboxes and delete selected button     

Bulk delete alternative process

1. create a custom menu
2. edit items that you want to delete and try to put them under 1 parent (or few parent items)
3. edit those parent menu items and reassign them to the custom menu 
4. delete the custom menu


Bulk edit

For bulk edit the https://www.drupal.org/project/menu_editor module is not yet ported to drupal 8 but it will be a good candidate if it gets ported