Group Classification and Microsoft Graph

I'm pretty sure the information is already available the time you asked about it. No need for a powershell.

Using the groups endpoint:

https://graph.microsoft.com/v1.0/groups

I can get the needed information. The following group in my tenant is listed as expected: 

https://graph.microsoft.com/v1.0/groups/0ea9da35-18d3-4c63-aa33-1daad3aa483b/classification

{
    "@odata.context": "https://graph.microsoft.com/v1.0/$metadata#groups('0ea9da35-18d3-4c63-aa33-1daad3aa483b')/classification",
    "value": "LBI"
}

Here is the documentation:

https://developer.microsoft.com/en-us/graph/docs/api-reference/v1.0/resources/group

If you are looking for all possible values you can check the specific settings (must be applied already to the tenant):

https://graph.microsoft.com/v1.0/groupSettings/

For my tennant:

https://graph.microsoft.com/v1.0/groupSettings/737eac5e-c369-43cc-bcbb-42dd764240a2/values

Result:

...
        },
        {
            "name": "ClassificationList",
            "value": "Intern,Vertraulich,Streng Vertraulich,Geschlossen"
        },
...