Recently, Microsoft released an update on the current adoption of .NET Standard by the development community. The primary reasons to upgrade to the .NET Standard 2.1 would be the inclusion of various new methods across the base class library (BCL) to support span-based APIs for writing allocation free code, and support for several new language features.
The .NET Standard is a formal specification of the APIs that are common across the existing .NET implementations for different platforms, and allow cross-platform development. The current specification, version 2.0, was released two years ago, supported in .NET Core 2.0 and .NET Framework 4.6.1 platforms (among others).
According to Immo Landwerth, a program manager on the .NET team at Microsoft and contributor to the Standard:
.NET Standard is for sharing code. .NET Standard is a set of APIs that all .NET implementations must provide to conform to the standard. This unifies the .NET implementations and prevents future fragmentation. It replaces Portable Class Libraries (PCLs) as the tool for building .NET libraries that work everywhere.
Using the standard allows a developer to create .NET libraries that can be consumed across the different .NET implementations. These libraries can also be distributed and consumed as NuGet packages. Cross-platform development with .NET Standards 2.0 is supported by Visual Studio since version 15.3.
The current adoption to .NET Standard was measured by data collected from nuget.org, considering (i) the top one thousand packages (ranked by the number of downloads) and (ii) all packages available at nuget.org. As of July 2019, .NET Standard was supported by 47% of the top one thousand packages and by 30% of all packages. The adoption rate has been measured since the release of the 2.0 specification of .NET Standard. A linear extrapolation of the adoption rate growth since them provides an estimate of 100% adoption by around 2022 - with a strong remark about the fact that this is a trendline prediction.
As part of its .NET library guidance, Microsoft recommends that all developers target .NET Standard:
With few exceptions, all libraries should be targeting .NET Standard. Exceptions include UI-only libraries (e.g. a WinForms control) or libraries that are just as building blocks inside of a single application.
.NET Standard is currently being updated to version 2.1, which will include support to newer versions of .NET implementations (including .NET Core 3.0, scheduled to be released next month). However, Microsoft states that all libraries should keep targeting the 2.0 specification, especially considering that .NET Framework 4.8 will not implement the 2.1 specification.
More information about .NET Standard can be found here. Also, Immo Landwerth keeps a curated list of short videos on YouTube showing how .NET Standard works, and how it can be used by developers.