SharePoint foldershare notification after expiry?

Site owners do get notified about expiring guest access, but not specifically about folder shares. To extend access, they must renew the guest’s permissions via the banner or admin center. 

You can use PowerShell (Get-SPOExternalUser) or Graph API to generate reports of upcoming expirations.

Keep external shares short-lived, renew only when justified, and avoid lifetime access. Best practice is to enforce short expiration windows (30–60 days) and require owners to renew only when there’s a clear business need. This approach prevents “forever shares” that can expose sensitive data long after the collaboration ends.

If possible, create a site dedicated to external collaboration so expiration management is easier.

Here are few articles which explain how to extend sharing - 

Manage guest expiration for a site - Microsoft Support

Does SharePoint Online send email notifications about expiring external collaborations to Site Owners or to Site Collection Admins or both? - Microsoft Q&A

External IT Knowledge - Managing Guest Expiry in SharePoint

Microsoft SharePoint: Guest Expiration | Information Technology Services (ITS) | Niagara College

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