Microsoft.Extensions.Configuration is combined with a core configuration abstraction under Microsoft.Extensions.Configuration.Abstractions that allows for building different kinds of configuration providers to retrieve key/value pair configuration values from in the form of IConfiguration.
Documentation can be found at https://learn.microsoft.com/dotnet/core/extensions/configuration.
The APIs and functionality are mature, but do get extended occasionally.
Microsoft.Extensions.Configuration is included in the ASP.NET Core shared framework. The package is deployed as out-of-band (OOB) too and can be referenced into projects directly.