WinHTTP has added support for HTTP/2 trailing headers in modern versions of Windows.

WinHttpHandler should set trailers into the HttpResponseMessage.TrailingHeaders collection in .NET Core 3+.

An idea that the gRPC team is thinking about is using WinHttpHandler to support Grpc.Net.Client on .NET Framework. For that to work the trailing headers would need to be available in .NET Framework, which doesn't have the HttpResponseMessage.TrailingHeaders API.

A possible solution that doesn't involve adding new API to .NET Framework could be to set the trailing headers collection as a value in HttpRequestMessage.Properties using a known key, e.g. __Http2ResponseTrailingHeaders.