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AllowNull Attribute in c#8 for netstandard2.0 and net451 raise compilation error [duplicate]

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Environment: vs 2019 v16.3.1

in c# netstandard2.0 and net451 project, I use c# 8 nulable reference with attributes.

Enable c# 8 in csproj:

    <LangVersion>8.0</LangVersion>
    <Nullable>enable</Nullable>
In myclass.cs, I use AllowNullAttribute:
     class My class {
       [AllowNull]
     public T MyValue {get;set;}
When building the project, I get a compilation error:
  
CS0246 The type or namespace name 'AllowNullAttribute' could not be found (are you missing a using directive or an assembly reference?)    ConsoleApp1 (netstandard2.0)
The documentation Attributes extend type annotations include these types
What is wrong in this code?
                There is no reference in c# 8 docs to exclude these frameworks, see my reference in the question.
– M.Hassan
                Nov 15, 2019 at 4:17
C# 8 (and therefore, by extension, AllowNull) does not seem to be fully supported in netstandard 2.0. 
For example, if you go to the documentation for AllowNull and select .NET Standard 2.0 as the version, you will get the following message:
  
The requested page is not available for .NET Standard 2.0. You have
  been redirected to the newest product version this page is available
Also see this Github issue. Namely,
  
Officially, C# 8.0 is only supported on runtimes that adhere to the
  .NET Standard 2.1. That does not (and will not) include .NET Framework