Class not showing up

[Varmod]

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.

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.

[HongGit]

@Varmod the API you are asking is part of the message security feature which it is not supported yet. Could you please share your usage scenario? Please also share any other APIs you need, and not supported/exposed yet. If it's currently supported scenario in WCF Client, along with missing APIs, we will consider exposing these APIs.