Hi
Thanks for reaching out. The app permissions that you have configured to query chat messages are correct. However currently there is no way to create application policies for chat messages to scope access. I would suggest you provide your comments and feedback on the Microsoft Graph Feedback portal.
Regarding OnlineMeeting.ReadWrite.All
permissions and the Meeting Artifact Endpoints administrators can use ApplicationAccessPolicy cmdlets to control online meeting access for an app that has been granted any of the following application permissions:
OnlineMeetings.Read.All
OnlineMeetings.ReadWrite.All
OnlineMeetingArtifact.Read.All
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Thanks.