When the .NET CLI was first created, all dotnet nuget * commands (and dotnet add package) would run NuGet.CommandLine.XPlat.dll as a child process. NuGet parsed this XPlat app's command line using a library that is no longer maintained.

NuGet.CommandLine.XPlat should migrate to System.CommandLine. This change should be invisible to users, unless there's a bug in the implementation, since the command line arguments should not change, just the library used to parse them.