Top 20 NuGet immutable Packages

Library for validating that .NET types are immutable.
Immutable binding library for Bridge.NET projects.
Roslyn powered analyzers for C# to support defining immutable types
The immutable class library is a set of libraries that consumes and extends System.Collections.Immutable
Get all the common units of measurement and the conversions between them. It is light-weight and thoroughly tested.
Extension methods used to copy and update immutable classes (as copy and update record expression in F#).
Immutable Json library
Json.Net serialization extension for Belt
MASBicudo Data Structures - Immutables, Continuous.
Json.Net serialization extension for Finalist
A base class for creating immutable composite user types
A base class for creating immutable user types
A lightweight, flexible and easy to use do/undo/redo implementation based on immutable objects for .NET.
Inheritable tuples with protected members, correct equals contract and optional cacheable hashcodes.
Package Description
Provides fast and memory efficient alternatives to the collections in System.Collections.Immutable. Currently just includes SchematiqList as a replacement for ImmutableList.
Package Description
Code generators for [Record]-marked types; generate boilerplate code for immutable record types. See project site for installation instructions and more details.
Analyzers and CodeFixes for [Record]-marked types. See project site for more details.
Conceptually immutable byte string type