SLA time goals setting lets you set time limits for each SLA configuration and choose the Exceeded actions for issues:

• notifying users in comments or via Slack;

• changing assignee;

• changing priority;

• changing status.

If you want to notify users about exceeded issues in comments, you can choose an option to notify one user (individual) or a group of users in comments about exceeded issues.

And in case want to notify users about exceeded issues via Slack, configure sending such reminders according to the instructions here.