a demonstration that by making a class sealed, it's possible for it to take a generic parameter of an internal type, and still have that class referenced by other assemblies

SealedInternal.cs

using NSubstitute;

namespace ConsoleApp
{
    public class SealedInternal
    {
        public void Run()
        {
            // this executes fine
            Substitute.For<IDoSomething>()
                .DoSomethingSealed<Thing>()
                .Returns(new SealedThingResultWrapper<Thing>());
            
            // this blows up with exception Castle.DynamicProxy.Generators.GeneratorException: Can not create proxy for type ConsoleApp.ThingResultWrapper`1[[ConsoleApp.Thing, ConsoleApp7, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]] because type ConsoleApp.Thing is not accessible. Make it public, or internal and mark your assembly with [assembly: InternalsVisibleTo("DynamicProxyGenAssembly2")] attribute, because assembly ConsoleApp7 is not strong-named.
            Substitute.For<IDoSomething>()
                .DoSomething<Thing>()
                .Returns(new ThingResultWrapper<Thing>());
        }
    }

    internal class Thing{}

    public interface IDoSomething
    {
        SealedThingResultWrapper<TResult> DoSomethingSealed<TResult>();
        ThingResultWrapper<TResult> DoSomething<TResult>();
    }
    
    public sealed class SealedThingResultWrapper<TResult> {}
    public class ThingResultWrapper<TResult> {}
}