Class not showing up

Not much a bug, more a request for help.
I'm porting some older code to netcore and I need to use the `System.ServiceModel.Security.MessagePartSpecification` class.
Given the github source code I would expect to find the class in the `System.ServiceModel.Primitives` nuget package, [file link](https://github.com/dotnet/wcf/blob/release/6.1/src/System.ServiceModel.Primitives/src/System/ServiceModel/Security/MessagePartSpecification.cs).

But the class is not there, I've tried other packages as well but I can't seem to find it anywhere. Could someone point me to where it is located?
The error I get is:
`CS0246 The type or namespace name could not be found (are you missing a using directive or an assembly reference?)`
Strangely enough some of the classes are showing up normally.

Currently I'm using the package versioned 6.1 since I'm not able to use the latest NET8 yet.