Admin only taxonomies?

When registering the taxonomy just set public to false, and it’ won’t be accessible on the front end. Just make sure to specify show_ui to true, otherwise it will use the public value and be hidden.

register_taxonomy(
    'my_taxonomy',
    'product',
    [
         'public'  => false,
         'show_ui' => true,
    ]
);

If you’re registering the taxonomy with a GUI from a plugin it should have checkboxes for these options.