Enable SharePoint Online Usage Logging
Please enable SharePoint Online usage logging on tenants or allow each tenant to disable or enable as needed.
We are unable to determine when SharePoint Online sites have last had user activity on it. This will help as part of our overall governance process and perform some necessary cleanup of unused SharePoint Online sites. Microsoft resource confirmed this solution is disabled across all tenants. A powershell command - Get-SPOSite -limit all -Detailed | select * provides inaccurate information as a backend process can run and impact the last modified date.
Dave Abbott commented
Usage/Popularity reports give an idea however our business wants to know which users have viewed certain pages/Document libraries or downloaded documents. I have worked round the document libraries however am surprised it's not a feature yet?