Those APIs still exist on .NET v4.5 of ServiceStack.Razor RazorFormat.
Razor on .NET Core is a completely new and different implementation where we no longer have control over the implementation of Razor and instead use MVC’s Razor APIs. We have some APIs .NET Core’s RazorFormat like GetPageFromPathInfo(), FindView() and RenderView() which will let you render to a Stream but for any other functionality you’d also need to use MVC’s APIs directly but I’d recommend taking a look at http://templates.servicestack.net which is a highly versatile templating language that’s well suited for Emails without the issues of using Razor.