<PackageReference Include="System.Configuration.ConfigurationManager" Version="10.0.0-preview.7.25380.108" />

System.Configuration.ConfigurationSettings

public sealed class ConfigurationSettings
Provides runtime versions 1.0 and 1.1 support for reading configuration sections and common configuration settings.
namespace System.Configuration { public sealed class ConfigurationSettings { public static NameValueCollection AppSettings { get; } public static object GetConfig(string sectionName); } }