We plan to add support for TLS 1.3 in 2019. Initially, this will be available on .NET 4.x, .NET Core and Xamarin platforms. Later versions will support TLS 1.3 on .NET 2.0/3.5 and possibly .NET Compact Framework 3.5/3.9 as well (although a plugin will be required). However, please note that older devices based on Windows CE and its derivatives might not be powerful enough, resulting in reduced performance of TLS 1.3 compared to TLS 1.2.
We have no plans to add support for HTTP pipelining or HTTP/2 in the near term.