After updating my solution to .Net 9.0, faced similar issue with Serialization Formatters library when serializing json data to a file using Newtonsoft Json library. Had resolved it by taking explicit reference of latest "System.Runtime.Serialization.Formatters" library in project even though it is not directly used.

Below is the error: System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime.Serialization.Formatters, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.

As per below article, BinaryFormatters have security vulnerabilities because of which "System.Runtime.Serialization.Formatters" library is obselete in .net 9.0 and removed from .Net 9.0 SDK. https://learn.microsoft.com/en-in/dotnet/standard/serialization/binaryformatter-security-guide

From Nuget Manager, if we add this library it downloads 8.0 version itself mentioning it as 9.0