You can clone a ServiceStack project, create a PCL project, link files, create and see what is missing. I am sure that there is a reason why PCL is no longer available, but if this happens to you, make a transfer request.
Since you are using MvvmCross, another option is to create a plugin for your API and then reference the ServiceStack libraries in platform-specific projects.
You can also use MclvmCross RestClient and make the same calls that the ServiceStack client will make.
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