When managing users and groups, you can enable a feature called Access Delegation. With Access Delegation, the User Profile Service sends an email to the user's manager (as specified in the user profile properties) upon the deletion of a user account. However, if the service is unable to determine the manager (for example, the manager field was blank in the user's profile), then you can specify a secondary site collection admin to add instead.

To configure access delegation, follow these steps:

1. Navigate to the SharePoint admin center Classic Features administration center (https://admin.microsoft.com | Admin centers | SharePoint | Classic Features).
2. Launch the User Profile Service application.

3. Under My Site Settings, select Setup My Sites.
4. Scroll down to My Site Cleanup.
5. Ensure the Enable access delegation box is selected to delegate to the account that was specified in the user's manager attribute. 
6. Under Secondary Owner, add a name from the user picker to be used as a backup:

7. Scroll to the bottom of the page and click OK.

The deleted user's manager will be delegated rights to the site upon user deletion. This change only affects new deletions going forward. It does not affect existing deleted user sites.