As a company, we have a newsletter for mandatory e-mails (eg. service and product updates) that we send to customers regardless of their e-mail preferences (obviously, as these are important messages for everyone).
Is Ghost able to facilitate this use case, or does it always allow readers to unsubscribe?
Ghost is not designed to be used for transactional emails - so there is always an unsubscribe link available in any newsletters that are sent.
As a workaround, you could maintain a separate Ghost instance for sending these types of emails - and just delete all members / import the list you want to send to each time you need to send out an important announcement. You would need to decide whether this seems like something that would fit with your audience and the type of emails you send 