Hello @BenediktSnow
Thank you for reaching out. I would like to confirm we can query last sign-in activity on user using Graph API select parameter, however you would need to use beta version and following query:
GET https://graph.microsoft.com/beta/users?$select=displayName,userPrincipalName,signInActivity
Following is the output screen on Graph explorer which confirms that it returns last login activity of the user:
I hope this helps.
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