What is the equivalent of IHttpControllerSelector for .NET 6?

We're implementing IHttpControllerSelector in our ASP.NET Web Api (.NET 4.5) for Controller Discovery because some of our Api Controllers resides on a different assembly and we have custom 'routing'(namespace) for it.

config.Routes.MapHttpRoute(
    name: "CustomApi",
    routeTemplate: "{namespace}/api/{controller}/{action}/{id}",
    defaults: new { id = RouteParameter.Optional }
);

Example:

• Assembly A Controllers will be called assemblyA/api/controllerA/get
• Assembly B Controllers will be called assemblyB/api/controllerA/get

And so on...

Then we implemented the interface to resolve the controller based on our criteria (code omitted for simplicity).

public class NamespaceHttpControllerSelector : IHttpControllerSelector
{
    public HttpControllerDescriptor SelectController(HttpRequestMessage request)
    {
        //This is where we resolve the controller based on the value of the 'namespace' route.
    }
}

Then while migrating our codes to .NET 6, I couldn't find a way on how to do this. I tried looking for the IHttpControllerSelector interface but it's only available to lower version of .NET Core but not .NET 6. Any ideas on how I can accomplish this same requirement for .NET 6? Thanks.