What does PrivateAssets=’All’ mean?

PrivateAssets is a metadata tag used to control dependency assets.

You might be using a dependency purely as a development harness and might not want to expose that to projects that will consume your package. In this scenario, you can use the PrivateAssets metadata to control this behavior.

Package references (PackageReference) in project files

In your case, unless you want to expose PropertyChanged.Fody to a consumer (i.e. you are releasing a library), setting PrivateAssets to All in your .csproj file will remove the warning.

<Project Sdk="Microsoft.NET.Sdk">
    <ItemGroup>
      <PackageReference Include="PropertyChanged.Fody" Version="3.3.1" PrivateAssets="All" />
    </ItemGroup>
</Project>