Hi Residentx10,
Yes, you are correct. Only E3 plan and E4 plan support uploading external data connection models (.bdcm files) to configure Business Connectivity Servcies (BCS) and the Secure Store. You can see more information about this by downloading Microsoft SharePoint Online for Enterprises Service Description.docx at the Microsoft download center.
For more information about BCS in Office 365, here is an article for your reference.
Introduction to Business Connectivity Services in SharePoint Online
@SharePointFrankOnMS, we appreciate you sharing the helpful information.
Thanks,
Maggie Li