I just switched from (an older) Microsoft.Bcl.Immutable
NuGet package to System.Collections.Immutable and was surprised to find all these new package dependenci
You are just seeing a side-effect of the Nuget package having to keep a lot of people happy. The package supports an enormous number of targets, it is proliferating rapidly as of late. I see support for Xamarin for OSX and iOS, Windows Phone 8.0 and 8.1, Windows Store, CoreCLR (the open source project), .NET 4.5, MonoTouch for iOS and Android and .NETCore (Silverlight).
These dependent packages just contain reference assemblies, the kind that are normally installed in your c:\program files x86\reference assemblies directory. The Nuget package doesn't take the chance that such a reference assembly might be missing and includes the whole kit and kaboodle.
After it is all downloaded, the package installer runs and adds the references you actually need in your project. Easy to see what happened, just open the References node of your project. If your targeted the desktop version of .NET 4.5 and up, the grand total of added references is one, just System.Collections.Immutable. Yes, you can remove them.