Undefined array key "delete"

For reference, this is reproducible only if the user does not have the access to delete the entity removed. In web/modules/contrib/inline_entity_form/src/Plugin/Field/FieldWidget/InlineEntityFormComplex.php:914 there is a check already

    if (!empty($entity_id) && $this->getSetting('removed_reference') === self::REMOVED_OPTIONAL && $entity->access('delete')) {

and the delete checkbox is not added if the user does not have 'delete' access. This, checking either for the access or if the array key exists is a valid case.
I don't know if tests are needed as this is a warning and a simple missing check that does not affect the end result. The user does not have access, the entity is also not marked for deletion.

Thx a lot, this makes a lot of sense.

But code is overly complex. Please use the more idiomatic !empty($form_values['delete'])