Consider targeting a lower .NET Standard version

[bdbai]

Is your feature request related to a problem? Please describe.
YtFlow is a UWP VPN app that includes an own-created Shadowsocks implementation. In the future, it may be adapted to use this project as a dependency.
In order to make YtFlow runnable on Windows Mobile family, the minimum Windows SDK version is set to 10.0.15063. Therefore, the maximum compatible .NET Standard version is 1.4 according to .NET implementation support. However, this project targets .NET Standard 2.1 which is impossible for YtFlow to consume directly.

Describe the solution you'd like
Consider including both .NET Standard 2.1 and a version at most 1.4 as the target frameworks of this project. For the API invocations that rely on .NET Standard 2.1 only, use conditional code as described in Target frameworks in SDK-style projects.

Describe alternatives you've considered
Would be better if Infrastructure becomes an independent project without any platform-dependent features (even Socket or SslStream) and leave only Stream-related APIs.

Additional context
See YtFlow/YtFlowTunnel

Windows Mobile

I think it's time to drop it...

[ohsorry]

.net standard is growing really fast, many APIs are waiting to be added.
Several APIs in this project require 2.1. I'm afraid it cannot be downgraded.