How about adding ModuleInitializer?

[kzu]

In the same vein as Nullable and this project, this attribute is missing in older TFMs and is required to create a module initializer:

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false)]
    public sealed class ModuleInitializerAttribute : Attribute { }
}

C# 9 proposal: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/module-initializers

It's already shipped too :)