Question Missing Assembly Reference errors while updating Unity 2019.4.22f1 LTS to 2020.3.29f1 LTS

We are upgrading our project to 2020.3 LTS version and we are getting missing assembly reference in our project. These errors are mostly from third party plugins that we have imported in our project and were working fine in unity 2019.

We are getting following type of reference errors:

Error CS0246 The type or namespace name 'AppsFlyerSDK' could not be found (are you missing a using directive or an assembly reference?)

Error CS0103 The name 'HelpshiftSdk' does not exist in the current context

We tried lot of solutions mentioned on forums for eg. https://answers.unity.com/questions/1728809/namespace-references-keep-breaking-in-visual-studi.html

Adding both the files (missing reference class and files which causes the error) in same folder works but is not a feasible solution as a file may contain multiple references to different classes.

We are using API Compatibility level .NET 4.X. Our project is built for android platform.