As of now this feature is not supported in Microsoft Graph API.
You can suggest a feature request at discuss Microsoft Teams.
Microsoft Graph API (Teams) :Provision a flag to show whether a message is read, or get the count of unread messages for Channels and Chats

Srinivas Kini
1
Reputation point
The Graph API for teams does not support fetching information about a chatMessage resource being marked as read or unread. Unlike Outlook, where a message resource is marked is isRead True/False.
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Meghana-MSFT 2,166 Reputation points Microsoft Vendor
2021-07-14T12:50:32.53+00:00