Hi @Martin Dye
Sorry I don't know much about .Net Core. I also didn't find any valid information about upgrading Microsoft.ReportingServices.Interfaces to .Net Core.
According to the official documentation, "The Reporting Services Extension Class Library is a set of classes, interfaces, and value types that are included in Reporting Services. This library provides access to system functionality and is designed to be the foundation on which .NET Framework applications can be used to extend Reporting Services components."
For more details, you can refer to this link: https://learn.microsoft.com/en-us/sql/reporting-services/extensions/security-extension/authentication-in-reporting-services?view=sql-server-ver16.
Best regards,
Aniya