Where is the 'subscription history' exposed?
The subscription history is shown in the Group tab of the Contact summary in as much as you can see Groups people 'were' in.

The the civicrm_group_contact table stores that people are 'removed' from the group, but the date that it happened is pulled from civicrm_subscription_history (as part of this function: \CRM_Contact_BAO_GroupContact::getContactGroup).