We would require minimum .NET Standard v2.0 and it would force us to introduce a range of breaking changes and significantly increase the system requirements. We'd like to potentially do this in the future, but it's some ways out due to these constraints (We have actually already done infrastructure changes to make it easier to potentially build for .NET Standard in the future).
Until then, you can use shared libraries instead to re-use your code across multiple projects.
Having said that, due to licensing restrictions you cannot use the ArcGIS Runtime on your backend (and you don't really need .NET Standard do that anyway).
Also consider that the .NET Runtime really is just a thin wrapper around a bunch of native C++ code. So just because you get .NET Standard libraries to compile against, if the native library for the specific platform you're running on isn't available (ie Tizen, Linux, MacOS etc) it still wouldn't work, as we're currently only shipping Android, iOS, Win32 and UWP native libraries.
Also see the discussion here: https://community.esri.com/message/688315-xamarinforms-are-pcl-projects-supported#comment-707268