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There is no official document available yet when this endpoint will be moved to production supported version. It is still in beta version and it's possible that it could be subject to changes without prior notice, however, there is no official announcement that indicates the removal of that endpoint. https://learn.microsoft.com/en-us/graph/api/intune-shared-windowsupdatestate-get?view=graph-rest-beta.
This issue could be related to a temporary bug.
I am testing in from my end and will be able to if that works from my end.
I am not seeing any other API we can use to get this property;-