Restirct user access to SPO Root SiteCollection

Hi,

From my experience, I see that there is an "implicit requirement," although it's not an explicitly documented requirement. The root site serves an infrastructural function as the entry point for all of SharePoint Online. Lack of access to the root site caused service issues; for example, the Graph API required permissions for the root site, and Viva Connections assumed that the Home Site was accessible to users.

Microsoft defaults to broad access (EEEU). Even in April 2019, as I recall, there was a thread about Microsoft accidentally restoring default permissions for this site when adding a new feature, indicating that functionality may be tied to its permissions.

The root site also cannot be deleted – its absence causes 404 errors and the inaccessibility of sub-sites. Personally, I would avoid limiting access to the root site and at least maintain "Limited Access" for internal users.

Links for more info: 

https://learn.microsoft.com/en-us/troubleshoot/sharepoint/sites/url-that-resides-under-root-site-collection-is-broken

https://learn.microsoft.com/en-us/sharepoint/modern-root-site

SharePoint root site permissions issue | Microsoft Community Hub

Root site permissions | Microsoft Community Hub

I hope I helped you gain a new perspective.😎