If you try to delete a permission set included in a permission set group that’s assigned to users, you now receive an error. You can only delete a permission set that isn’t assigned to users either directly or through a permission set group. Previously, this action wasn’t blocked, which allowed users to be assigned to empty permission set groups without enabled permissions.
Where: This change applies to Lightning Experience and Salesforce Classic in Contact Manager, Group, Essentials, Professional, Enterprise, Performance, Unlimited, Developer, and Database.com editions.
Get Notified Before Deleting Permission Sets Assigned to Users (salesforce.com)