It's a pity 2.1 will not be implemented by netfx 4.8. I wonder if there will ever be E netfx with netstandard 2.1. Sadly things are not looking good for standardization - on one hand we have new netstandard without netfx and on the other aspnet.core without netstandard ( https://github.com/aspnet/AspNetCore/issues/3753 ).
(Also, it looks like ASP.NET Core still should be considered .NET Standard 2.1 compatible, but yes the "upgrade" to .NET Standard 2.1 is fascinating because it loses compatibility with netfx.)