Hi Dave Hughes,
Thanks for reaching out. You can find information on Graph API for Microsoft Business plans here under the Content Serices section.
To answer your question yes Graph API is supported with Microsoft 365 Business Premium. However please note some Graph API endpoints will require a specific subscription. You can check the specific Graph API endpoint reference document for more information. https://learn.microsoft.com/en-us/graph/api/overview?view=graph-rest-1.0
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Thanks.