If subscription alerts are enabled, a patron can sign up for subscription alerts from opac-serial-issues.pl. Although opac-alert-subscribe.pl contains code for unsubscribing, there is nothing in the interface which offers the patron an unsubscribe link. opac-serial-issues.pl should be aware of whether a logged-in user is already subscribed, and show an unsubscribe link instead of a subscribe link.
This works for me on current master. I can subscribe and unsubscribe, the links show accordingly.