因为 .NET 标准已被弃用，取而代之的是 .NET 5。

While .NET Standard 2.1 will be the last version of .NET Standard, .NET 5 and all future versions will continue to support .NET Standard 2.1 and earlier. You should think of net5.0 (and future versions) as the foundation for sharing code moving forward.

请参阅 Immo 的帖子： https://devblogs.microsoft.com/dotnet/the-future-of-net-standard/

文章特别提到了 Blazor，因此值得一读。