How does GDPR affect reminder and confirmation SMS and emails?

Confirmation SMS and emails, as well as reminder messages, are considered ‘transactional’ messages, rather than marketing. It is part of the service your client asks of you, so you have a legitimate interest, and it is also in the client’s interest to get these messages. However, should a client come in and ask you to stop sending these messages, you should remove their consent to receiving reminder messages. Asking for reviews is also considered a transactional message, as it is part of your service.