We are introducing a new advanced capability for SharePoint Administrators to restrict SharePoint and OneDrive sites to specified users. With this feature, SharePoint administrators can restrict site access to specified users using Microsoft 365 group or AAD security groups. Users not added in the specified group(s) will not be able to access the site even if they were previously granted site access.
This message is associated with Microsoft Roadmap ID 163991
This feature is currently rolled out and is available via SharePoint Admin Center or SharePoint Management Module.
SharePoint administrators can leverage this feature to reduce the risk of oversharing or permission sprawl within their organizations.
What you need to do to prepare:
There is nothing you need to do to prepare. For more information, please visit this documentation.