Support → Email Notifications → Ticket Notifications
You can create a new notification which will be sent whenever any SLA policy time is over. You can create a new notification at Support > Email Notifications > Ticket Notifications > Add New > Trigger: Out of SLA.
Setting up conditions is optional for this notification.
You can edit the subject and email body as per your requirement, and also add recipients.
This notification will be sent for all the SLA policies, you can create multiple notifications for SLA by adding conditions to differentiate between them. For example, if you want another Out of SLA notification only for the Marketing category then please create a notification with the same trigger and add a condition category equal to Marketing.